I think you are on a bit of hiding to nothing really mate, the logical step for a 4x4 is to go to a 2wd rear diff, 7.5" instead of the 4x4 7", 108mm drive shaft flanges instead of 100mm, much stronger & a direct swap apart from taking 1/2" off the 2wd case, or swap to a 2wd rear beam. The 4x4 diff is 3.52:1, where the 2wd is 3.64:1 , it can be swapped out without meddling with the centre diff as it will be able to compensate for the tiny difference without damaging the center diff, it is what is fitted to my Escos.

Rear diffs generally are not a problem on 4x4, you would be better off looking to sort out the front diff that is notoriously weak!

If you do want to mess with rear diffs, the easiest way would be to get a new drop gear made up, although I don't imagine it would be cheap?