Aw4 swap question

guys I need some help here. My sister has a 1991 XJ with Aw4 and a 231. The trans is going out. I have a lead on a 1989 xj Aw4 with a 242J. I know the 89 has a 21 spline count but I just want to see if I can swap it and the tcase in long enough to get her original one repaired.

I can get the whole 1989 XJ for 300.

You should be able to. You'll also need to swap the tcase bezel and shift gate but that's easy. You may have to do some wire splicing to get the tcase and transmission connected up but electrically they are functionally equivalent.

So whats the issue with the tranny?

She has 250,000 miles on it.

Rig specs
31" x 12.50 15r procomp AT's on black 15x8 Rims
I6
3.73 Gears
3" Rusty's off-road lift


Its slipping and she isn't able to maintain hwy speeds. Also I think the converter is heating the motor up b/c her oil temp will drop and water temp goes up.

motor now has about 60k on it. Driving around town is fine if she keeps it under 50mph.

She is going through a divorce and is strapped for cash right now and on top of that this is her and my nephew's only way around.