Hey Peter I dont think the clutch will kill your tranny, its more likely the tires and gears your running that would kill it. You changed to larger tires, but have you changed the axle ratios to reflect the switch to larger tires, and the way you use the truck. The larger tires are doing more to "kill" your trans than the clutch will ever do.
From reading the posts the simple answer is the dual friction.
The long complicated drawn out answer is that the drivetrain is mismatched to the use.