You can get them with the Chrysler 8.25 rear axle or a D44 on some older ones. These are both decent axles. The 8.25 currently can only be geared as low as 4.56 if that matters. I'd stay away from a D35 since the others are available.
I like the 4 sp Auto in mine (AW4). There's a guy on NAXJA selling a manual shift kit for it pretty cheap, which also allows you to lock up the torque converter offroad 8)
THe newer XJ's (97-01) are supposed to be more prone to driveline vibes, but I have no problem with mine on 4" lift and a TC drop. They stopped using the high pinion D30 front axle in 97 to make more room for the larger exhaust header. I'll take the low pinion in place of the notorious crack-prone old header.
You'll probably get advised by some to get a 96 or earlier because of vibe issues (which is often actually the front DS), but 97 and newer have better axle U joints, a better header, a little more power, a nicer interior, and will most likely have the larger 29 spline 8.25 rear (the pre-97 8.25 axles had smaller 27 spline shafts). The newer XJ's have their advantages and driveline vibes are easily correctable with TC drops, adjustable control arms (with come with any
good lift kit).
Mine has the open carrier with 3.55 gears. Most will have that but some have the limited slip rear, which would be nice if you luck into one.
SOme came with a full-time Xfer case, don't know alot about them but they're out there. Mine's part time just like your old Jeep and I like it that way.
Can't think of much else off the top of my head but I will help answer anything I can.
www.NAXJA.com has a great XJ tech forum, but search before you ask as they are not as tolerant to newbies as we are here on Jeepz
So... you gonna build it up too??? :twisted: