could be the cable, could be the gear in the transfer case. easiest thing to do is to undo at the transfer case and see if the end will spin in your fingers, and if it will, if you can see the speedo, if not have someone look at it and see if the needle moves. if it won't spin in your hand, then disconnect from the speedo and see if the insides of the cable is in one piece, and if you can spin it and if you spin one end you can see the other turning. if the cable is ok, plug it back into the transfer case and then have the end which is supposed to go into the speedo where you can see it and watch to see if it spins when you drive it (protruding from under the dash for example. if you know the cable is ok from the first few steps, if it will not turn with the other end in the transfer case, then your speedometer gear in the case is also toast. if it does turn, then it is the speedo, pull it out of the dash and see if it will move by installing a small screwdriver in the back of it where the cable connects. if not, likely it is frozen up inside. i just had this problem with my 79. I soaked the speedo in about an inch of PB Blaster in the cap, balancing the speedo on it witht he shaft doen in the liquid, and then gently worked it unitl it broke free after a day and a half of soaking. put it all back together and it worked.
those were the steps i went through to determine where the problem was and the eventual remedy. of course there is always the possibility that it could be any of those or more than one (cable, speedo gear, or speedo) at the same time. Also possible that if it is the speedo, it is so locked up you can't get it to unlock without breaking something. then there is always EBay