You might pull the speedo cable and drive with it poking out of (or under) the dash and watch to be sure the cable spins as it should when the truck is moving. I might even grab it with my fingers while driving, to be sure it turns strongly and doesn't slip or skip. If the cable turns as it should, then it's likely internal to the speedo head.
Colo Springs, CO
'78 FJ40, '89 FJ62, '98 4Runner