Sounds pretty bad locking up so hard like that. If its staying parked with plans to swap, pull the cover and see what you find. Maybe you will be surprised and its something much less than you'd expect.
Regardless, just swapped my d35 with 3.73s last week. To avoid modifications including welding, your only swap options for the rear end would be a d35 or d44a from another ZJ. The only thing that will matter is the gear ratios, all the rear ends are the same size, no matter what motor, trans or t case you have, so don't sweat that so much. Just search Craigslist and car-part.com for similar year d35's or d44a's with the same gear ratio as your front. As mentioned, if you don't already know it, the gear ratio should be stamped onto a metal tag on the axle itself, typically found on the diff cover. Once you source a new rear end with matching gears, you just gotta swap it out.
The swap is pretty straight forward, just take your time on everything. Here's s simple break down:
Before lifting, loosen all control arm bolts ahead of time.
Lift it as high as possible by the body and support with jack stands, so the axle can droop.
Remove wheels and driveshaft from rear end, and let the axle droop.
Disconnect shocks, sway bar links, track bar at the axle, and upper control arms.
Remove brake calipers and zip tie to body, remove rotors.
I had to swap brakes on mine, but that's a whole other thing.
Disconnect the misc stuff from axle like diff breather, abs lines and brake lines & brackets. Carefully lift an zip tie outta the way.
Disconnect lower control arms and pull old axle old.
Slide new axle under, and replace in reverse order.
Really not bad, just a matter of taking things off and putting them back. Replace what you break along the way. Make sure you cover everything in PB blaster or oil a week in advance, every day until you do the swap, you will see why...
Right now, just focus on finding yourself a new rear end with correct gears, and maybe lower mileage (although mileage doesn't necessarily matter on a well cared for rear end, might even find one with newer bearings...). Expect to pay between $150-$250 for a used rear. I wouldn't pay much more than that honestly.
Hope that helps a little, I'm no pro, but was forced to do this swap myself last week with little time to prepare and perform. Not a bad job overall.
'99 WJ Laredo 4.0L, 174K, NP242 (Current)