... had this in my Grif too, and ended up having to replace the speedo.
Speedo just sometimes doesnt move, then clicks back into life all of a sudden. Was fine on way home, 2hrs use, and fine until I took the car out this afternoon. Anything obvious to look for?
There is also the sensor that is located by the diff and reads from a toothed wheel. This needs to be close to the toothed wheel (but not touching!) for the speedo to work. Somethimes it just needs a little adjusting
you often get excess movement in the diff output shaft, the extra movement causes the toothed wheel to eat through the end of the sensor. replacing the sensor sorts it for a short time until it happens again. there's a fix though