You may be able to get away with using the same pinion angle if no SYEs are involved... but I would set up your perches correctly. The distance between the perches on XJs and MJs is different.
Cut the XJ perches off, and bolt everything together somewhat snugly. Make sure your axle is centered, and set your pinion angle. Tack the perches on, pull the axle back out, and finish weld them.
MJs had different springs for 4wd and 2wd.. the 4wd ones sat a bit higher, that's it. You don't need the 4wd springs to have 4wd.