sorry im talking about spline count on the input shaft, but i guess everything 91 and up was the 23 spline? not sure. i just know when i did my swap the input shafts were the same length but the donor was a 21 spline so i had to swap it for my 249 23 spline shaft. i swapped over the whole planetary gear set, it was a 1993 to an 1989 case so the ring and sun gears matched up.
still though, his 91 case is going to have the different internal gear cut since the change over was in 94, so he will need to use the front half of his 249.
1993 Jeep ZJ. 4.0