Just disconnect it under the truck and cover the port on the transmission to keep it clean. Use your gps for a speedometer until you get it fixed.
After about 50 years the grease is dry as a bone inside the speedometer and the cable. Might need to disassemble to lube it up for it to work right again.
It doesn't take much drag to make the needle bounce like that, could be either or both. Its not going to hurt anything to disconnect it at the transmission, and the problem might cure itself when the weather is warmer.
