Properly set up, "2" wheel drive is surprisingly capable. If you modify the differential so that power is actually getting to both wheels simultaneously.
What are your expectations of the rig?
I've driven a LOT of pickups, spud and grain trucks, and plain ol' station wagons with locking differentials through some serious mud, snow, and tough terrain.
Were I in your shoes, I'd put a locking differential in the rear of that 'two wheel' drive, and use it to pull a play rig to the rock pile. XJ's are still reasonably inexpensive, there's a ton of 'em out there and available, why modify a cool little station wagon?
I think that if I ran across a two wheel drive XJ, it'd end up with a stout V8, and go play on the street. Make a helluva sleeper, wouldn't it?