Since it's one of the most commonly used transfer cases in late-model Jeeps, the New Process 231 has a lot to live up to. Not only does it have to scoot every one of those Jeeps around on all four wheels, it also has to put up with the extreme driveline angles that inevitably occur when said Jeeps receive massive tires and suspension kits to better their 'crawl-ability.
While that might not be a big worry to longer-wheelbase 231-equipped vehicles, it is for YJs, and TJs. In fact, considering the 231's more-than-adequate Low-range of 2.72:1, its ease of shifting, and its low weight of 65 pounds, these are really only a couple of problems with this venerable unit, and all have to do with the rear output shaft.
The rear output on the NP 231 is a slip-yoke design, which means that it accommodates all of the inward and outward movement of the driveshaft. This is problematic for two reasons. First is that this design dramatically increases the overall length of the transfer case, which, in turn, dramatically shortens the length of the driveshaft you can run. With short-wheelbase vehicles, this means the rear driveshaft has to run at extreme angles, creating excessive driveshaft vibration and frequently broken U-joints (even with double CV joints).
The second problem with the slip-yoke design usually becomes apparent after you install a suspension that provides good articulation. When you are moving through an obstacle and the rearend really begins to droop, the slip yoke slides outward to accommodate the movement. The catch is that it can only slide out a few inches before it falls right out of the housing and your driveshaft hits the ground -- irritating and embarrassing.
So what's that remedy? That depends on your definition of remedy. The suspension kit manufacturers addressed the problem by offering spacers to lower the transfer case and maintain the stock driveshaft angle. A cheap and easy fix, yes, but what happened to all the ground clearance you were hoping to gain from the lift you just installed? If big tire fitment was all you were after, no worries; but, if you're hitting trails, that transfer case is going to be scraping all the way.
So what to do? Two words: tailshaft conversion. Take that NP 231 tailshaft, lop off a good chunk of it, and there you have your extra length of driveshaft. Not to say you should lop it off yourself, of course. Luckily, there are already a handful of companies that'll do it for you.
Most kits indicate a gain of as much as 5 inches in driveshaft length, and that, combined with a CV joint, makes for zero vibrations. Also, with the addition of a fixed-end yoke (offered with most kits), the leak-prone, slip-yoke rubber seal is eliminated. The following should fill you in on the rest. Now get busy, it's time to go 'wheeling.
 Advance Adapters From the...  Advance Adapters
From the company that could probably make an adapter to make you and your mother-in-law click comes a short-shaft Fixed-Yoke kit that adds 4 inches of rear driveshaft length on Jeep YJs and as much as 6 inches on TJs. Advance Adapters manufactures a new one-piece tailhousing for the NP 231 and combines it with a new 32-spline output shaft that's said to be more than 50 percent stronger than stock. |  JB Conversions The NP 231...  JB Conversions
The NP 231 Slip-Yoke Eliminator Kit from JB Conversions features a new, heavy-duty, 32-spline mainshaft with an 18-percent-larger spline diameter and a 54 percent increase in torque capacity. The new shaft can accommodate as much as a 1-ton output yoke to match larger rear axle assemblies and is manufactured to New Venture Gear specifications. The kit also includes a custom one-piece rear bearing housing that is chemically pressure-treated to ensure against leaks. |  Tom Wood's Custom Drive Shafts...  Tom Wood's Custom Drive Shafts
Offering tailshaft conversion kits for both early and late-model NP 231 transfer cases, Tom Wood's Custom Drive Shafts also includes a new heavy-duty driveshaft with each package. Both kits convert the tailshaft to a fixed-yoke/flanged output design and are said to decrease the net U-joint angle and allow for a vibration-free driveshaft. |
 Currie Enterprises Leaders...  Currie Enterprises
Leaders in the custom axle business since 1959, Currie Enterprises has expanded into the Jeep market with a vengeance, offering everything from skidplates and bumpers to suspension and, you guessed it, NP 231 tailshaft kits. The Currie offering is said to shorten the stock T-case length by 4 inches in Jeep YJs and allow the use of a longer CV-style driveshaft. The design increases the angle of the transfer case so that you can lift your Jeep without lowering the skidplate, thus robbing ground clearance. Kits are also available for TJs. |  Rubicon Express Rubicon Express...  Rubicon Express
Rubicon Express offers its heavy-duty slip-yoke eliminator kit for NP 231-equipped Jeep YJs, TJs, and XJs. The kit includes a new, larger-than-stock tailshaft and tailhousing and all necessary housing seals and bearings. Rear driveshaft length is said to be increased by 4-1/2 inches and a CV driveshaft can also be installed. |  TeraFlex Tera's short-shaft/slip-yoke...  TeraFlex
Tera's short-shaft/slip-yoke eliminator kit is designed to work in stock NP 231 T-cases or in units equipped with the company's Low231-series T-cases. The new, heavy-duty shaft withstands the increased torque generated by extreme gear reduction and lengthens the driveshaft by 4 inches. The kit includes a larger output bearing and rear housing and requires the use of a CV-style driveshaft. |
 Collins Brothers The Collins...  Collins Brothers
The Collins Brothers Heavy-Duty Yoke Kit includes a new, U.S.-made tailshaft that's manufactured to New Venture Gear specifications and a one-piece rear bearing housing that's molded and machined to exact specifications. The new shaft features 32 splines as opposed to the factory 27 splines and has an 18-percent-larger spline diameter. The kit is said to add 4-1/2 inches of rear driveshaft length to Jeep YJs and 6-1/2 inches to that of TJs. |  Skyjacker A winner of SEMA's...  Skyjacker
A winner of SEMA's Best New Off-Road/4-Wheel Drive Product 2001, Skyjacker's heavy-duty Slip-Yoke Eliminator Kit is said to reduce the transfer case tailshaft length by 6-1/2 inches on TJs and 4-1/4 inches on YJs. The kit includes a redesigned transfer case mainshaft with a shorter and beefier 32-spline unit, allowing for a 54-percent increase in torque capacity. |  Novak Conversions The Model...  Novak Conversions
The Model 231 Tailshaft Conversion Kit from Novak Conversions is designed to strengthen your NP 231 transfer case while eliminating the poor slip-yoke design. The stock output shaft, housing, and yoke are all replaced with superior components manufactured by Novak. As a result, the new, shorter shaft allows for better driveshaft angularity, which is crucial in lifted Jeeps. For more information, contact: Novak Conversions Inc., Dept. 4WDSU, P.O. Box 3367, Logan, UT 84323, (435) 753-2513, (877) 602-1500, www.novak-adapt.com. |