I don't know of the diff itself will fit, but it does need a carrier to take it out of the axle or it falls apart. The axles themselves do swap, but i believe the Mk3 axle is slightly narrower. We've done this many times on the LWB lutons, and it's never noticable, though it may be on the vans.
Mk3 diffs will fit mk2 axels BUT it should be from the same type of axel ie. mk2 F type axel will take a diff from the mk3 F type axel.
The crownwheel and pinion i think will swap between most diffs (but you need to check the ratios)
Ive just rebuilt the diff on my '95 six stud single wheel axel, the crownwheel and pinion from a mk3 F type axel fitted with no problem even though the diffs are totally different
any external size difference that would effect the prop length? asking this as i need the prop shortening about 2-3" and would like to avoid it if poss.any of you ever shortened your own ?
internals are the same and as already stated tho must be same axle type.
there will be slight differences in length between axles in relation to flange position but it should only be minor due to the bearing style and should be taken up by the sliding joint in the prop
and yes it is noticable on the vans fred .... it looks almost anerexic and it also runs the tyres fairly close to the inner edge (poss enough to rub if tyre goes flat)