How difficult is it to convert a 2wd cherokee to a 4wd if I've already got the transfer case, front axle, and driveshaft?

you will also need a 4wd transmision and misc linkage.

Not all that hard if you have all the pieces and parts. Those things are going to nickel and dime you to death. You may have been better off buying a wrecked XJ and stripping it. It sure would have been cheaper.

No no, I HAVE a wrecked 4wd XJ....I'm BUYING another body to swap my drivetrain into. Just need to know how hard it would be to swap MY 4wd drivetrain into a 2wd XJ

It's a unibody. Aint no such thing as a body swap. You're altering a 2wd XJ into a 4wd XJ. You're going to get a new VIN out of this swap.

Technical-wise, you're going to have to move a lot of heavy parts and require jacks and stands. You should be able to transtition everything with just nuts and bolts.

How bad is the wreck?