Unscrew the cable from the trans and put a cordless drill on it and run it kind of slow in reverse. Might need a helper to look at the speedo and see if it is working. If it is working then it may be the adapter box thingy if your truck has one. If it isn't working, try to pull the cable out. I cant remember if they go in at the trans or at the speedo. If they go in at the trans, pull it out till you find the end, it should be square on the end. If it is round or looks ravelled then the cable is broke. IF it is supposed to go in at the speedo (there's a brass stopper thing on it) and you pull at the trans end, if it comes out it is broke. If it dont come out, it still may have the square end broke off and the brass keeper is stopping the cable from coming out. Getting to the back of the speedo and the clip that holds the cable on is a real bitch, so you've been warned!! One other thing it could be if that all checks out is the plastic speedo gears in the tailshaft of your trans.
