stu, youre a bit wrong there.

MT75 = the sierra one
MTX75 = the FWD jobby.

the X has nothing to do with 2wd/4wd/fwd

ps i think it would be a great idea to slap the st170 box in if you had an unlimited budget, but since we all live in the real world, i'd say stick to an old ERST gearbox with LSD. it'll do the job you want, and have a nice slipper (which you probably wont need TBH)

ps whats the plans for the XR2?