What sort of driveshaft angles are you running? To develop a side force on the output housing requires that the output shaft and the driveshaft be at an angle. (You don't seem like the type to have installed it with an excessive angle...) Is it possible that under heavy load when the chassis rises the slip yoke is pulling far enough out of the output housing to allow centrifugal force to throw the yoke sideways? If so, it seems that the intitial force would be amplified by the engine torque to create a huge sideways force.
