Biggest And Baddest Rear Axles For Your JK

JK rear Axle guide

By now you’ve probably seen the recent article on Figuring Out What Front Axle To Use In Your JK Wrangler. To follow up on the front, we’ll now cover the toughest bolt-in rear differentials to put into your JK Wrangler.

There are multiple companies that build bolt-in differential assemblies for JKs. We’re going to cover the top names and their most hardcore units, and a couple extra options. Follow along for the lowdown.

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Currie Enterprises

Currie Enterprises is one of the leading differential builders in the world for all sorts of motorsports. They have been gaining experience for over half a century. Their biggest and most wicked bolt-in differential assembly for the JK is the RockJock 70. This thing is monstrous. “As tires continued to get larger,” Brian Shephard of Currie explains, “differentials needed to evolve to larger formats to keep up. JK Jeeps, for example, are very heavy and commonly on 40-inch tires. That takes a considerable size differential to withstand that abuse.”

Currie RockJock 70 Features

  • 100-percent bolt-on
  • 10.625-inch ring gear
  • Currie exclusive 35-spline pinion gear R&P set, as opposed to the stock 29 spline pinion gear
  • 65-45-12 ductile iron housing center
  • 3.5-inch diameter DOM axle tubes
  • 0.375-inch thick wall DOM tubing
  • AR400 steel skid plate
  • 35 or 40-spline chromoly 4340 axle shafts
  • Billet steel full float housing ends
  • Works with the stock JK brake assembly
  • Compatible with factory ABS and ESP
  • Five, Six, and 8-lug options
Modeled after a high pinion Dana 70 differential found in four-wheel drive Top Kicks and other medium-duty trucks while incorporating the popular and effective features of the Rock Jock 60, Currie stepped up the bar big time. The RockJock 70 sports a 10.625-inch ring gear, the internal diameter of which is big enough to put a typical Dana 30 ring gear inside of without touching the 70’s ring gear.

The high-pinion housing is designed to reduce driveline angle and to help get the driveshaft out of harm’s way while being extremely strong. When designing the casting, the 65-45-12 ductile iron housing center, Currie only used the original high pinion 70 as a reference. A whole new design was created that included extra webbing and a load bolt to put pressure on the ring gear, avoiding deflection while under load. Under the belly of the beast you’ll find a drain plug and the AR400 steel skid plate to allow the unit to slide over rocks with ease. The tubes are built from 3.5-inch diameter and 0.375-inch thick wall DOM tubing.

The RockJock 70 is set up to use stock JK brakes, nearly directly bolt on. Only a few small modifications are needed, or you can purchase the pre-modified brakes from Currie. The Chromoly 4340 axle shafts are available in 35 or 40 spline. The axle shafts set up into billet steel full float housing ends and allow for ABS and ESP sensors to be retained. The RockJock 70 is the largest high-pinion differential in the aftermarket and is available in five, six, and recently released, 8 lug configurations.

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Dynatrac

Dynatrac came into being because more strength was needed. A Dana 44 had broken and a Dana 60 was needed. Over the years, the differential assemblies coming out of the Dynatrac design and manufacturing facilities kept getting stronger. Now, Dynatrac offers the ProRock 80, the biggest and baddest of them all. The low-pinion housing was designed to give far more clearance, handle more torque and be lighter weight.

The axle uses 11.25-inch Dana 80 ring gears, comes with 4-inch tubing and can be had with either 0.25-inch or 0.375-inch thick wall tubing. Dynatrac uses a design they’ve created, the Dual Sump/High Volume/High Pinion Angle Oil Modification for instances where 11 or more degrees of pinion angle are required, with 40-spline 4340 chromoly axle shafts coming standard. The full float modular iron wheel hubs, chromoly heat treated spindles and serviceable bearings were designed to utilize heavy duty J8 Military 13.9-inch vented disc brake assemblies with a standard 8-lug bolt pattern. As with all Dynatrac assemblies, the ProRock 80 works with factory Jeep ABS and ESP systems.

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G2 Axle and Gear

G2 Axle and Gear has been building internal components for differentials for many years. Gear sets and axle shafts were their primary parts. Along the way, they jumped into the replacement housing game. In their current offerings, G2 sources a RockJock III 60 housing, with all bracketry welded on, from Currie Enterprises. The housings are then loaded with a locker, G2 gear set, Timken bearings and G2 chromoly axle shafts. The RockJock III center section is built from ductile iron, uses the Currie AR400 skid plate and uses 3-inch diameter tubes.

Though the G2 uses the same core housing as the Currie RockJock III 60, the benefit is the bang for buck value with the G2 Axle and Gear internal components. This unit comes with a 5 lug bolt pattern and is designed to utilize the original JK brakes as well as the original ABS and ESP sensors.

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Mopar/Dana

Dana built the original differentials all these others are based upon. As an OEM supplier, Dana designed and built assemblies for all sorts of vehicles and is the most popular brand of OEM parts. The diffs under your Jeep right now, right off the factory floor, are Danas. For OEM applications that required more heft and would see far more abuse, Dana built stronger assemblies, like the Dana 60 and 70.

In recent years, the aftermarket world has surged with demand for heavier duty differential options. Military J8s utilize the Dana 60 rearend; Mopar and Dana now offer it as a strong replacement differential assembly for JK Wranglers. The Ultimate Dana 60 is made in the USA and uses a full float design and 35 spline axle shafts made from SAE-4340 nickel chromoly steel.

The Dana differential uses a Spicer 9.75-inch diameter ring and pinion and has 3.5-inch diameter axle tubes that have 0.390-inch thick wall. Each unit comes with 14.09-inch diameter brake rotors with dual piston calipers, 8 lugs and is built in the USA.

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Teraflex

Teraflex has been building aftermarket components for Jeeps for many years. Much of their contribution to the Jeep world has been performance suspension systems. Then, while keeping up with their suspension innovations, they decided to focus on replacement differentials. Teraflex offers several different replacement JK differentials and their top dog is the CRD60, a full float, and high pinion rear differential for the JK Wrangler.

The ductile iron center has additional gussets over a standard Dana 60 which help eliminate load deflection. Internally, the housing features large heavy duty bearing caps to assist in the rigidity department. The CRD60 uses what Teraflex calls a “skimmer.” This skimmer picks up oil and keeps a reservoir full, that reservoir is for the front pinion bearing, ensuring it never goes dry — a huge benefit when it comes to high pinion angles and steep inclines. The stout differential uses 3.25-inch diameter tubes that are 0.375-inch thick. It comes with new brakes, 8 lug bolt pattern and retains all the OEM safety systems.

Other Options

In addition to bolt-in differentials, there are companies like Artec Industries that build truss kits for 1-ton truck differentials. The truss kits are built from 0.25-inch plate steel and are designed for additional brackets to be welded on, using the truss as a reference, so the differential can bolt-in to the rearend of a JK. Artec offers kits for a Corporate 14 bolt conversion as well as a Ford Super Duty Sterling 10.25/10.5.

Artec Industries' truss system allows you to weld brackets to a Ford Sterling 10.25/10.5 so it can be installed in the rear of a JK.

Artec Industries’ truss system allows you to weld brackets to a Ford Sterling 10.25/10.5 so it can be installed in the rear of a JK.

The downfall to conversions such as these is that it is just a bracket kit. A strong one but it does not include necessary components for the ABS and ESP system conversion. Those are left to the end user to sort through. But that may not be a major concern considering the Sterling differentials have been used in Ford trucks since the 1980s and are very reliable. The large gear set, wide range of ratio options, and big axle shafts with a large spline count make it an extra strong contender.

Left: Ruff Stuff Specialties fabricated axle housing. Right: SpiderTrax fabricated axle housing.

Another option is to check out companies like Ruff Stuff Specialties and SpiderTrax Off-Road. Both companies build tough fabricated differential housings that are designed for a Ford 9-inch third member to drop in. The Ford 9-inch, while not as big as many of the others on this list, has been used in Jeeps for many years. For an even bigger step, there is a Ford 10-inch drop in third member that can be installed as well. Many Ultra 4 race buggies use fabricated housings for the tough demands and poundings of high speed and crawling.

While tough as nails, neither company builds a bolt-in differential for JKs. All axle brackets would need to be sourced or custom built and then welded on. Not a job for the faint at heart but do-able.

Go Big

Ultimately there are lots of options available. Brand new, awesomeness with a warranty from several manufacturers and stout OEM components with some extra brackets alike will do wonders under nearly any rig. JKs are very much a multi-use vehicle. There are many ways one can be built, used, abused and cared for. Just remember to think long term when you are spending a good chunk of change, you wouldn’t want to drop that kind of scratch on new parts only to outgrow them in a short period of time.

Article Sources

About the author

Jake Headlee

Jake's passion started at a young age wrenching on cars with his Dad. Obtaining that glorious driver's license sparked his obsession with grease and horsepower, and the rest is history. Soon, he was a general mechanic and suspension specialist, and currently designs and modifies products for the off-road industry. Jake enjoys rock crawling, desert racing and trail running, and writing in his spare time.
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