This is the second time the splines have stripped out of the rear planetary any ideas?
Background on the truck its a 2002 standard cab long bed. Cam stall (3600fti) and afew other bolt ons. 4.8 that is making 350hp at the wheels. Seem to happen 1st to 2nd shift.

 

I would be looking closely with the fitment between the reaction shaft and the rear planet. Or replacing them both. As transmission builders we don't see this often. You could step up to a 5 pinion GM AC Delco planet. Reason being is the materials used are much better. Hope this clears up your issues. Vince

 

How was the trans set up???

Is the line pressure maxed???

Is the accumulator blocked off???

Are you backing up and shifting from reverse to forward fast or vise versa with out coming to a complete stop???