yeah they will work. basicly from the doors forward MJs are almost exactly the same as XJs. i have RK long arms that the site says might not work with 1990 and earlier models, mines a 88. no issues what so ever.
and yes depending on wear of the rear leafs its anywhere from 4.5-6.5. i know some people on here had to go a hair more then that with coils/spacers up front to level it off. that was only a hand full of people IIRC.
also check out barnes4wd.com for longer perches when you do the swap to help fight that axle wrap. spring under sucks but gets the job done. just expect to beat and bang the leaf plates/shock mounts on everything! plus in mud they really do act like road graders and take up too much ground clearance IMO