Sorry about my newbness, but I have a 91 xj that has t-case drop that I'm going to replace with a 231 with sye ( hack & tap) out of a 99 xj.
The new t-case has a drive shaft that comes with it. Is this a straight swap or am I going to run into problems?

 

I'm not 100% but the speedometer or the other wire that plugs into the t-case might not match up. My buddy wanted to swap his 89 tcase with his 01 but couldn't due to the connectors being different but i'm not sure which one it was. i could be wrong though so let someone else chime in.

 

the wiring will be an issue but that's about it and can easily be sovled. Next up is the original t-case a 231? If so direct bolt on no shifter adjustments, if your current t-case is a 242 then you will have to also modify the shifter.

 

no it wont fit, the input splines are different. the 91 is 21 and the 99 is 23.

 

not sure, but i seem to recall the spline count is different...

 

1990 is the cutoff year for 21 spline, 1991 is 23 spline input.

it will fit, and as far as any electrical connectors...just swap the sensors if you don't want to deal with any other hassle.