You may have a bad cable from the trans or bad gear at the trans connection. Disconnect the cable at the trans and spin the inside rod and have somebody watch the speedometer for any responce of the needle. If no responce then disconnect the cable at the rear of speedometer and insert a small screwdriver and spin and watch for any responce of the needle. If it works here then you have a bad cable. If it doesnot work here then you have a speedometer problem.
4x4s have the speedometer on the transfer case, not the trans.
the gear inside is steel, not nylon.
hard to find a correct new 4x4 cable, only ones sold in the parts stores are for the 73-up and have a clip instead of the nut at the back of the speedometer. i have modified those type by carefully removing the ends and installing the correct ones from the old cable.
65 crew was by here looking for one, he said he could have cables made up at a shop in Fairbanks so you might check around by where you live.
1966 F250 4x4
1964 Rambler Ambassador 990
Rest in peace departed Slick family members
Cam Milam
Lesley Ferguson
Steve Lopes
John Sutton