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Super-Duty Jeep Transfer Case Conversion

We Upgrade The Transfer Case on Our ’06 Jeep Wrangler TJ

Stuart A. BourdonPhotographer, Writer

Much of the project plan for this 2006 Jeep Wrangler TJ has been to maintain an old-school vibe and keep it somewhere in the middle between dirt road cruiser and rockcrawler. We wanted a simple and reliable build that we can have a lot of fun with. As of this report, it sports 4 inches of Currie lift, Currie Antirocks front and rear, Rancho RS9000XL shocks, and 15x8 Mickey Thompson wheels. At the moment, a set of 33x10.50R15 BFGoodrich KM2 Mud Terrain tires are wrapped around the aluminum wheels. It can take 35s if we want. All that change created a chain reaction, as do most modifications to any vehicle. The lift demanded new driveshafts, and a slip-yoke eliminator kit was needed for the TJ’s New Process 231 transfer case to allow a longer-than-stock rear shaft for better pinion and U-joint angles to help eliminate vibrations.

After browsing for a few hours we settled on the Tom Wood’s Custom Drive Shafts Super-Duty 231 slip -yoke eliminator kit for the NP231 transfer case. Why not a low-range gear ratio conversion? The NP231 offers a versatile 2.72 low range and direct drive in high range. With the 4.0L straight-six engine, six-speed manual transmission first-gear ratio of 4.46:1, and a ring-and-pinion gear swap that will come with the new locking and limited-slip differentials (that’s a whole ’nother story to come soon), we can run 33- or 35-inch tires, and the 2006 TJ will have all the giddyup it needs to go exploring.

What’s the difference between heavy duty and super duty? Well, the Tom Wood’s Super-Duty 231 conversion kit comes with the manufacturer’s unique rear output flange that allows the installation of a 1210, 1310, 1330, and 1350 series driveshaft with a conventional two-joint CV (double-Cardan) setup, or a 1410 series conventional two-joint driveshaft, all without changing the flange. As is very common, a Jeep goes through many evolutions throughout its lifetime. Who knows what further modifications may be made down the trail. With this setup, we’re not limited to one type or size of driveshaft, and the flange also offers a convenient spot for a park brake rotor if that’s ever in the plan.