The transmission is the same, but the tailpiece and output shaft are different in design and length from the ones designed to hook up to a tranfer case. Also, 1991 is supposed to be thyear when AW4 tranmission output shaft went from 21 spline to 23 spline. In other words, you may have to swap the input gear (23 splien) on the 1992 transfer case to accept the 1990 21 spline AW4 output shaft. I cannot remember which year is the break between the cable-drive speedometer used on your 1990 and the later systems: have a look at this part on the 1992 to check, and look at the wiring harness- you may have to match the connections across the years if the connector changed (and lots did when jeep went chrysler parts). The wire colors never seem to match, Renix (1990) to HO generation.
Edit: I just found an old message I saved about this: author (not recorded) stated he had an 87 Cheorkee with the stock renix motor and he swapped trannys from a 91 jeep to get the 23 spline output. "All wires were the exact same on both trannys so I just swapped everything, it was plug and play" His only problem was accidently knocking the vaccum pipe of the MAP sensor- easy fix! Sorry I can't verify this.