If its temp dependent then I would be looking at the diff or the tranny and if you say the trans oil is good and doesn't smell burnt then I would focus on the diff.
Have checked the oil in the diff? Its possible that you have a bad bearing in the diff or the tranny.

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'85 F250 6.9L diesel 2wd XL XC LB Centurion, Gone
'95 Mercedes E 300 Diesel W124.131 with a OM606.910 and just over 200k on the clock, needs top end engine work and a transmission.
2000 S500 W220 with a 5.0L V8 gas power.