The NP231 is one of the most commonly used T-cases in current-production 4x4s. Dodge Rams use the NP231D. GM S-series trucks use the NP231C, and Jeep Wranglers and Cherokees use the NP231J (although there are three variations of the 231J). The simple and efficient design of the 231 makes it strong and provides excellent interchangeability between the various models.
If you've stared at your 231 long enough, you already know that it uses a slip-yoke at the rear and that the output shaft and housing are long, which can cause driveshaft-angle problems after you've lifted your 4x4. Short-shaft kits (also called fixed-yoke kits) alleviate both those problems by changing the output shaft, output housing, and yoke style. Kits are available from Currie, JB Conversions, M.I.T., and Rubicon Express, and each varies in its design and advantages. The main difference is that the Currie, M.I.T., and Rubicon Express conversions use a modified factory output shaft, which keeps costs down, and JB Conversions uses an unmodified larger-diameter shaft and bearing that substantially increase strength. Driveshaft lengths vary among kits, but you generally gain 4 to 41/2 inches, which is a big help for short-wheelbase vehicles.
Replacement Planetary Gearsets
All NP231 T-cases come with 2.72:1 low-range gears, which is accomplished through a planetary gearset, just like in an automatic transmission. The planetaries are interchangeable among all 231s; the only variation is a slight change in tooth profile between early and late boxes. A huge strength gain can be obtained, however, by swapping in a six-pinion planetary from the NP241 T-case used in 3/4-ton trucks (available from JB Conversions). According to New Venture Gear, the torque rating of the 231's three-pinion gearset is 600 lb-ft, and the 241's six-pinion set can handle up to 1,400 lb-ft.
If strength alone won't tempt you to swap the planetary gearset, how about lower low-range gearing? Six-pinion 4.0:1 or 3.5:1 gearsets are available from 4 to 1 Manufacturing for the NP231 and NP241. Gearsets from 4 to 1 have one-piece, billet-steel planetary housings that are much stronger than the factory powdered-metal, two-piece housings. Nevertheless, torque ratings for these gearsets aren't as high as for the six-pinion 2.72 gears (but are higher than the three-pinion sets) because lower gears multiply torque more, reducing the amount of torque input that parts can tolerate before breaking.
Larger Hubs and Chains
NP231s use a chain to transfer torque to the front output shaft. In all but the Dodge 1/2-ton T-case (NP231D), this chain is 1 inch wide (the NP231D uses a 11/4-inch-wide chain). This wider chain can be swapped into the other cases as long as you also swap in the wider drive sprockets.
Engaging into four-wheel drive from two-wheel drive is handled by a splined sleeve that engages a hub--all the power transmitted through the T-case travels through this sleeve. The sleeve and hub are pretty tough and almost never fail, but these pieces, along with the shift fork from an NP231D, can be installed into an NP231J case to substantially increase strength. This becomes an issue if you've swapped in a more powerful engine or have laden your Jeep with 1,000 pounds of extra gear.
Neutral and 2-Lo
A two-wheel-drive Low position reduces the frequency of three-point turns on the trail, since it stops the plowing effect that you get in four-wheel drive, especially if you have front and rear lockers and big tires. While Dana 300 owners can use twin sticks available from several companies, NP231 owners must replace the range fork and sector with new ones from JB Conversions to alter the movement of the internal sleeves of the T-case.
Finally, '94-and-earlier NP231s don't have a true Neutral position for trouble-free towing. When in Neutral, the input gear of the 231 separates from the front and rear output shafts, but the shafts are still connected. A special sector (available from JB Conversions) solves this problem. With a true-Neutral kit, the rear driveshaft spins independently of the rest of the drivetrain. Since the T-case oil pump is driven by the rear output shaft, lubrication of the T-case isn't a problem.
And that's the rest of the story.