did u check the bearings in ur brake seat and rear diff case? ive had issues like this before, all looked ok on the bench, but once the motor gave the bearings in the seat and diff some torque, they werent supporting the topshaft properly. like wise ive had the same with the clutch bearings too - just cos they seem ok in ur hand, doesnt mean they are ok when used.

always worth replacing the lil buggers every now and again, i do em every gallon or so on the whole truck - hub bearings being the quickest to wear out, followed by the diff bearings, then the clutch ones, then the brake seat and 2 speed ones, the steering post bearings seem to hold out a little longer tho. in my experience at least... was just a thought.

ebay usually has some cheap sets of tmaxx metal sealed bearings, theres a few u wont use tho - 6x10x4mm or something, but hey, u get to sell em on...

ps - anyone using alloy clutch shoes? if so id advise spending a little more on the clutch bearings as they get pretty warm with alloy shoes...