a couple things come to mind,
1. did you change your gears for the 36's? that adds a stress point.
2. did you rebuild or build up these trannys for your mudders and hard driving or did you just get 2 other old tired out trannys.
3. did you put the new t/c in correctly when you swapped the other 2 trannies?
if it was done right there should have been no reason to have to swap it out the 2nd time. let alone the 3rd.
as far as a swap, find a big block manual donor truck and start stripping parts.
pedal brackets, trans, clutch, flywheel, floor pan for shifter, computer, coloum, find and disable neutral safety switch.
ps. big tires with wrong gears and slipping trans = horrible mileage