Does the clutch bell spin? if not your clutch might be melted, this would cause the trans not to spin either. The only other thing it could is a mis-adjustment of the brake actuator on the throttle servo. Check all of those linkages to make sure that they are adjusted right. Also pull the rear diff out and turn it upside down. There is a hole in the bottom of it that you can look in and see if any of the ring gear teeth are broken. If nothing looks broken, just regrease it really well and put the diff back in. Let me know if any of this helps. If its not any of these problems I will try to come up with some other things to check.