The clutch isn't your problem. Could be some hot spots on the flywheel or pressure plate due to slippage but I doubt it since it only does it pulling a load. Does your truck have a 2 piece rear driveshaft? There was a TSB for launch shudder on trucks with a 2 piece driveshaft that recommended checking the rear driveline angle and replacing the tower that supports the carrier bearing. I'm thinking there were several different height towers offered to achieve the correct angle of the rear shaft. Also check your you u joints thoroughly for binding or wear. Especially the rear and middle (if equipped) on the rear driveshaft.
