Modified TD5 blown 2 rear diffs

My friend has a modified disco td5 running 285 75 16's.
The story is..
We used it for a while and then whilst giving it some abuse the rear diff blew. We replaced this with a second hand diff and took it to a pay and play day, and the rear doughnut exploded! Limped it home running only fwd and done a solid rear prop conversion.
Now after pretty much only road miles the diff has failed again, and we now question whether it's just the diff was going to go anyway (as it's a second hand part) or if the big wheels have caused it. I know someone who runs similar sized tyres on a standard diff with no issue.

Ive the same size tyres on my 110, only had it 3 months but not had an issue. Dod a p&p yday with no diff issues

Larger tyres increase the gear ratios overall. So in lower gears using lots of power it is obvious more strain is put on the diff trying to turn larger wheels against load.