i had a big long tranny/transfer choice explanation written up for ya but the computer ate it...
so once again,
Using the pages that the above reply mentions, you can change output gear receptor in your transfer case over to 23 spline, and then you will be able to swap in either the T-5 or the AX-15.
ck out: http://www.off-road.com/jeep/tech/xfer/np231.html
which explains: "There are versions of this (your) chain driven transfer case that take 21 and 23 spline input shafts. The 23 spline version is used behind the AX-15 and the 21 spline is used behind the AX-5 and the BA 10/5. The output for the front driveshaft is on the drivers side. Low range is 2.72 and high is 1.00. If you own a 21 spline NP231 and you want to swap in a 23 spline transmission, you can either look for a cheap 23 spline NP231 at the junk yard or go to your local Jeep Parts Dealer and order up a new input shaft part #5300-6085. This is the part you need to convert your NP231 to 23 spline input."
once done, you can throw in a T-5: "The T-5 was used in CJs from 1982 until 1986."
or the AX-15: "The AX-15 was first used in Cherokees (XJ) and Comanches (MJ) with the 4.0L engine. In mid 1989, the AX-15 was used in Wranglers (YJ) with the 4.2L engine and it continued to be used with the 4.0L engine. The first year of the Grand Cherokee (ZJ), the AX-15 was rare option only with the 4.0L engine. The AX-15 is still used in TJs and XJs with the 4.0L engine"
Now, start hitting junk yards. Most cars/Jeeps do not get sent to the junkyard because of manual transmission problems. The full-size jeeps will be more common than the swb. A self-pulled junkyard tranny could go for as little as $100. Be careful, don't get burned. Get the top cover off if possible, take a decent look inside, no holes, or other oddities. A few weeks ago i almost pulled and paid for a T-18 with no first gear.
Next, either throw it in, or mildly rebuild it (gaskets, seals, blocker rings, not too difficult).
The next option, more unlikely, is to check out and decipher: http://www.off-road.com/jeep/tech/Trans.html
Take out both tranny and tranfer, and build it up.
Best and cheapest bet, change the NP231 transfer part and dig up or find a T-5 or AX-15. Think junkyards, insurance settlers, manual tranny reuilders, and online bulletins, nice people online with too many Jeep trannies.
1979 CJ-7: 304, T-150, much is my rebuild
1985 CJ-7: 258, T-5, for Peru Excursion
1989 XJ: has seen every state except Hawaii
In search of lost paths and distant locations...