Its possible the speedometer head locked up at some point, and in turn stripped the transmissions gears. It would depend on if the cable was strong enough and if the gears were already worn.
Does the odometer ever change, it's direct gear to gear driven off the cable, the speedometer needle is magnetically driven off the cable. Do the gears turn in the transmission when you rotate the tires, have you watched them to verify?
What year of truck is this?
Joe
