I think the brakes are your culprit. It sounds like thats all that needs to be adjusted! The truck should roll freely when no brake is applied on the transmitter. I would suggest centering up the trim on your radio and popping the head off of the throttle/brake servo and putting it back on with the throttle at idle. If the brakes are engaged at this point then you need to adjust your brake linkage until there is no pressure on the brake linkage with the carb. set on idle.

The nut on the spur gear serves a different purpose. That adjusts the amount of torque transferred from the engine to the drive terrain. If you loosen up the nut, it will allow the gear to spin more by slipping. Its there to help lessen the amount of shock load transferred to the gears and diffs.

Take a pic of your diff if you can! These trucks are so big, they are inherently hard on parts... I wouldnt write the truck off yet.