for an easy direct bolt in raid a dana 44 or a chrysler 8.25 from an xj. if you feel like customizing go with the dana 60 or chevy 14 bolt. the way i figure it why waste time on modding a ford 8.8 or 9 rear when you can go all out and be sure your rear will be safe.
now on a personal note I hate fords and don't want anything i don't have to have on my xj from ford.

if you go as far as doing the rear end your best bet would be to get both rears from a chevy (14 bolt) the tranny and transfer case. then that way there will be less mods to worry about later,and a stronger drivetrain. go for the np203 if you can find it, it is a gear driven transfer case thus stronger.