There has to be something wrong with you breaking diffs like that! I've never heard, outside of competition, of anyone breaking so many, so easily, and actually none in competition either!
I'd remove the transfer box and check/refurb it, make sure especially that the output bearings and seals are in good fettle with no undue movement. Check also the bearing housings and that the bearings can seat flat and true. Similarly check the axle tubes for straightness, if one is bent even slightly the bearings are always out and might be putting undue pressure on one side of the diff. Check the hubs and bearings in them for same. Any bearing housing that's suspect might be causing an issue.
Then check the diffs or get them refurbed so you know the pre-load and backlash are set as good as they can be. Slightly tight will whine, but is arguably safer than loose as loose can introduce shock loads when the slack's taken up!
Not trying to teach you to suck eggs, you might well have done all this, but it seems to me that it's possibly something out of line rather than built wrongly for it to go wrong so many times!
'course, if you're thrashing it, spinning wheels and letting them touch down at speed when stuck that could be it too ..