51" from inside of both brake backing plates. As you know the axles shafts on the 8" axle are very strong the weak point is indeed the 8" ring gear. And although it holds up very well on a Toyota mini truck it may not be the best choice for a heavier xj. And not all 8" axles are the same, the stronger ones are the v-6 third member (more ribs on the center section). Not even all Tacoma axles are the same. Wont get into all of them on this thread. You would be better of finding an 8.8 from an explorer. I forget which years come with disk brakes but they are plentiful and pretty cheap.
Ed