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.