Chip - D449k is probably right. If the speedo drive gear spacer is missing there is nothing for the spacer sleeve to squeeze the drive gear against. Check out this manual for your truck, you can see the part #15 that must be missing.
Hope this helps, Bug
http://chevy.tocmp.com/1948_51truckshop/51ctsm0707a.htm


1951 Chevrolet 3100 AD Pickup