Least mods - wheel lug/pattern match, gear ratio match to front, parking brake lines, shock and leaf mounts, and pinion yoke. For a complete bolt-in, you'd be better to locate another Jeep rear axle from the same year/model. You could also look for other model matching years, but sometimes mfrs make little changes that wouldn't make it a complete bolt in.
Early model Wranglers (YJ) came with a D35 axle that was little stronger than late model versions. It has flanged axle shafts rather than c-clip axle shafts.
The Explorer 8.8 (95-01 w/disc brakes) is a great upgrade from a stock rear axle. For an install, you'll need to find a gear ratio that matches your front axle, purchase a flange adapter to your ds ujoint to mate with pinion flange, relocation of shock and leaf mounts, and I believe shorter parking brake cables. If you can't gear match, then you'll need a gear swap - upwards of $200 for labor, $150 gears, and $100 for install kit. Then with everything open for the gear swap, look into limited slip/locker ($250-500), new axle bearings/seals ($50), and diff cover protector ($50-150). The ds adapter is some $30 if that. Shock and leaf mounts will vary in cost very little (keep it simple), but they will require grinding off the old mounts and welding in the new mounts. You could use your original parking brake cables, but then you'd have to 'adapt' them to work with the parking brake mechanism of the axle.
Another potential axle is the Cherokee (XJ) 8.25. Again, look for a matching gear ratio. Since it was a Jeep axle, you may find that easier. Your parking brake cables may work with the 8.25 axle or the axle cables work with your YJ. You will be required to relocate the shock and leaf mounts. The ds ujoint should fit the yoke of the 8.25. You should make sure you get the 29-spline version (later years model - earlier years have 27 spline but there is aftermarket support to upgrade).
Ditto on XJ D44 axle.
And this is just for a simple rear axle swap with no plans of upgrades in suspension, tires, wheel changes, and matching the front axle. So investment wise, best to look at what you want to do with the Jeep in the next couple of years, how big a tire/wheel you want to go, how hard a trail you want to do without breaking, is it to be streetable/daily or trailered most of the time, suspension changes, etc. Lots to consider and weigh. Money spent to upgrade is going to be nearly the same cost now or later, but spend it now, you'll be adding to the costs the matching front axle and suspension upgrades....
