Possum,
If you can. Unscrew the driven gear from the transmission. This is the best way of seeing what you have.
When I got my transmission the speedometer wouldn't work either. I am switching from the 3 speed column shift to the floor 4 speed. I found that someone had taken the back housing off of it (I have a 53 1/2 ton with the enclosed drive shaft) and left two of the bolts in the housing. When they put it back together the bolts got between this gear and the gear of the transmission. Short story is that it messed up the driven gear. I hope this is not your case.
These driven gears are not impossible to find. I found mine at a swap meet in Fort Worth. You can always check at the salvage yards and the 'older' transmission shops.
Best of luck.
