DSRTRAT - The first thing I would do is hold the spur gear tight and push the truck on the ground to see if the wheels turn. This should tell you if the problem is the spur gear. If you have a clutch problem start the truck and hold the spur gear and rev it up and see if the spur gear receives any power. The best way to check the clutch is to take the bell off and inspect it and see how worn the shoes are. I've also had oil get in the bell and the shoes slip.

A related story:
I buddy of mine had problems with his MT with the .15 in it and he had a stripped button and didn't have enough compression to push the truck. New button and it was fixed. I never experienced a truck that would run but wasn't strong enough to drive.