If you buy the right parts, everything
will be bolt-in.
for the simplest swap get:
2wd MJ with the engine you want and the trans type you want.
4wd XJ of same era (87-90 or 91+) with same engine and trans.
and I would add to it:
WJ lower control arms (they will be in much better shape and have a bend to clear tires)
2" spacer pucks up front and Chevy drop shackles out back, or a full lift kit (2wd are a bit lower than 4wd and if you're going to do a lift, now's a good time)
that'll allow you to keep your rear axle, avoid any wiring issues, and the only thing you'll need to farm out is a shorter rear driveshaft (I had my 2wd shaft shortened for $30 at a local shop).
One caveat, if you live in a high-rust area, it makes everything that much harder.
Now, you can increase the difficulty a bit and convert using other years of parts or other models' parts, but if you do your homework and plan ahead (get a good floor jack, stands, etc) it can be done in a driveway over a long weekend by yourself (it's been done). It's a LOT easier with 2 people though.
-88 MJ; 4.0L, ex-2wd, TJ ax15/231, 78 Ford D44 w/spool, 9" w/posi, 33" trXus MTs, custom roll-bar/bumpers/etc.
Also an 86 MJ Metric ton and 2 '90 MJs
Projects/wheelin' pics: picturetrail.com/petermontie
My email: firstname.lastname@example.org