TOKKA

25-50 mph in reverse ... that's some hard core wheelin bud.

IMHO, keep the diagnosis simple and work towards the harder stuff. Check CV boots for tears. Check bushings for cracks. Grease anything with a Zerk. Check that all bolts are tight. Check heat shields around exhaust. If you have skid plates, make sure they clear all exhaust and suspension components. Check tire balance. Check brakes for any trapped stones.

Service Tranny. Replace fluids in both pumpkins & TX case.

Even if it's none of those things: 1. should do it every few years anyway & 2. you may locate the real problem at the same time.