Rear end problem x 2

[Bick in New Zealand]

Plenty of oil in the diff. I drained it and couldn't find anything. Oil was nice and clean with no filings. I pulled the top off the transmission to have a look and everything looked fine there. I initially thought it may have been the park brakes, but the skid marks on the road were pretty even on both sides. I will pull the brakes off tomorrow and see if anything gives there.