O.k I have two more questions for you. Will the bell housing of the AX15 bolt right up to my 88XJ? and my other question is; Are you saying that i need to get the 91YJ's driveshafts and put them on my 88XJ and get 88XJ's driveshafts and put them on the 91YJ?
Thank you so much for all your input. You guys are a big blessing.

Yes, the AX15 bell housing will boltup on 4.0's and 4.2's.
No, there is no way you will swap driveshafts without modification. Too much difference in length. I don't know how much difference in length there is between the two trannies but I believe the Ax15 is longer. You MIGHT have to change the length of 4 driveshafts if you do the swap. Most people would not swap a BA10 into a Wrangler. They are notoriously weak. (Also expensive to repair and hard to find in junk yards)

Today i took out the AX15 out of they 91YJ. It doesn't have any identification numbers on it. It had one number on the top of it which is: 53005360. Do you know what that number means? Anyway i put the AX15 right next to the BA10 to see the size difference. They looked like the same length but the AX15 looked bigger (bulkier). For the most part they looked similar but i noticed some differences like some of the wire hook ups where at different locations but other than that they looked similar. The AX15 had about a 1 inch plate inbetween the transmission and the transfer case. And the BA10 was split down the middle length-wise just like you said. So all that confirms that i'm actually dealing with a BA10 and an AX15. Not that i didn't trust your info but i had to know for myself b/c for all i know the previous owner could have put another tranny in. Anyway, since both of them looked like they were the same length it looks like i shouldn't have a problem with the driveshafts not fitting.
Here's a couple of more questions for you. Do i need to take all the wires that hook up to the transmission/transfer case and swap them as well or should i be able to use the same wires on the different transmission and transfer case? I believe i have to swap out shift levers and transfer case levers and linkage and all. Is that correct? One more question, Do you think the cross-members in each vehicle will bolt right up to their new transmissions?
Sorry for all the questions, this is the first time i've ever done this. I've done alot of other work on cars but i've never messed with transmissions until now. I always like to learn by experience but i figured since i found this Jeep forum i might as well get a little knowledge to make things easier on me. I'll keep you posted on my progress. Thanks for everything.

I'll try to answer what I can. Bear in mind I have never done a swap like you're doing. (lIke I said, Nobody swaps BA10's in place of an AX15). You should also start another thread (about 4 posts ago) since this has nothing to do with this thread. But anyhoo, first, no, I don't know what the numbers mean but I suspect they are a casting number. It is not important. As far as I know all AX-15's are the same. If the trans. are the same lengths there should be no problem with driveshafts. I'm not sure about all the wires you speak of. There is the backup light switch with one wire on my AX15 as far as other wires, figure out what they are for. The 4wd light switch is on the front axle on my 91 YJ. there are no other wires that I recall on it. You're not confusing vacuum hoses with wires are you? The shift sticks will stay with their respective transmissions, the T-case shifters are vehicle specific so they will have to stay with the vehicles. There is a good chance the crossmembers will be ok, the mounts obviously are different and have to stay with their original crossmember. There may also be an issue with the speedometer cable if the two Jeeps have different gear ratios in the axles. If they do, the tail housing along with the speedo gears should be interchangeable on the t-cases. The 92 wrangler should be 3.07:1. I don't know about your XJ, a lot of them have 3.55:1 gears. <img src="/forums/images/graemlins/cheers.gif" alt="" />
PS: I don't trust my info most of the time! <img src="/forums/images/graemlins/cheers.gif" alt="" />

Now I have the BA10 bolted up in the 91 YJ, but i'm having problems with the t-case shifter. I know you said the t-case shifters have to stay in thier vehicles and i see why you said that but the t-case shifter linkage won't go on the transmission/transfer case. HELP!!

I'm gruntin' for ya! That's about all I can do from here without knowing what the problem is. Hard to help from "it won't go". Is the bolt pattern different? Case position different? What's the deal? <img src="/forums/images/graemlins/confused.gif" alt="" />

The bolt patterns are different. The t-case shifter for the BA10/231AMC bolted on the inside of the 88 XJ but there's no where for it to bolt on the inside of the 91YJ. The T-case shifter for the AX15/231J bolts on top of the AX15/231J. Thats why it won't work. Plus the location of the t-case shifters are different. The t-case shifter of the AX15/231J is directly to the left of the gear shifter. The 91YJ was made for that on the inside and the 88XJ was not. The t-case shifter for the BA10/231AMC is diagnally down and to the left of the gear shifter. The inside of the 88XJ was made for that and the 91YJ was not. If i put that on the inside of the 91YJ i believe i would have to cut out room for it then i would have to figure a way to bolt it on the inside.

The t-case shifter plaste for the AX15/231J bolted on top of the AX15/231J. The t-case shifter had two bolts and the gear shifter plate has 4 bolts. But the left two bolts of the gear shifter plate is the same two bolts for the t-case shifter. The bolt pattern is square.

The bolt pattern for the gear shifter plate of the BA10/231AMC is diamond shape.So i can't bolt the t-case shifter plate to the BA10/231AMC b/c of that.

Plus i cant swap linkages for the t-case shifter without rigging it up some how.

In other words the t-case shifter and linkage is vehicle specific as well as transmission/transfer case specific.

I believe a need a t-case shifter and linkage specifically for the 91YJ with a BA10/231AMC. I don't know if they make that b/c its unusual for a 91YJ to have a BA10/231AMC.
I will also need a t-case shifter and linkage specifically for the 88XJ with a AX15/231J. They should make that b/c that isn't unusual.