Most likely has the least desirable transmission ever found in a Jeep- the Peugot BA-10. The x-case behind it will have a 21-spline input shaft and will not bolt right up to later transmissions without swapping in a 23-spline input shaft.
BA-10s are super-easy to spot. Take a look under the Jeep, and if you see a seam running lenghtwise down the bottom of the tranny... you've found it.
On a more positive note: BA-10s can hold up decently for daily-driving duties. The BA-10 in my Comanche has 117k miles on it and has not given me any problems. I got the Jeep with 106k miles on it, so there is always the possibility it's been replaced, rebuilt, or at least had the (PITA internal) clutch slave cylinder replaced once in it's life....