If you drill the holes larger and get brake line adators it is bolt on. so some modificaiton required.
What calipers are you running... you dont need to upgrade the MC when running stock axles. 1tons and disc front and rear, you will "need" to or you will be double pumping at every stop.
If you're running stock discs front/drum rears, a dual diaphragm with same year MC will improve your brakes. Based entirely upon reading numerous threads, I'd suggest the late XJ ('96?) booster and MC. You only need to change your combination valve if you switch to rear discs. While I've read write-ups on installing the ZJ and WJ boosters, and they seem to be good, there seems to be a potential problem with angles, if you want to swap in a latter Dodge Ram MC (along with much bigger calipers than stock XJ).
See Dingo509, above: RE Vetteboy and Doge MC.
I'm running my stock 95 XJ MC w/o abs. It stops my d60 front (dual pistion) and rear 3/4 chevy calipers (rear discs on an eaton) as good as or better than with the stock axles.