Speedometer not working

It could be the mechanical cable or the drive gear inside the transfer case. Does the odometer work? The easiest test for the head and cable would be to connect a drill to the drive end at the transmission, then run the drill counterclockwise while someone watches the needle. If the speedometer works then the head and cable should be okay. In that event You would need to remove the speedometer drive from the transfer case by removing the screws that secure it. Then look at the drive gear and the driven gear for any damage like stripped teeth, cracks, internal stripped gear. If the internal drive gear is damaged it can be replaced without removing the transfer case. If the driven gear inside the adapter is damaged it simply pulls out for replacement. It is common for both of them to wear to the point they stop working. New they look like the image, used they will have the middle worn way down. The driven gear will be worn as well. To get replacements you need to know the colors of both gears.