Unscrew the cable from the transmission, chuck the cable into your battery drill, rev it up and have a helper tell you if the speedometer is working.
If that works, reconnect the cable and jack the truck up off the ground, set it on jack stands, disconnect the cable from the speedometer under the dash, start the truck, put it in gear and see if the cable is spinning. If it's not spinning then the problem is down at the transmission.
If the first step didn't work, pull the cable out of the outer jacket and see if it's broken or has a bad end. Universal cables are available and can be cut to length and slid back into your original jacket.
You could also have a bad speedometer head...
Hope this helps,
Mike B
