The problems that I could see was the odometer cable that went from the transfer case to the dash.  It had too many tight bends due to the cable being a little too short. The bends in the cable caused tension with core that rotates with the speed of the vehicle.  What broke on my Hummer was the 90 deg. elbow at the transfer case (three times!), and the last time was the gears within the transfer case as seen below.

The electronic sending unit AMG sent to me would not fit directly on my transfer case, my service manual did not show me just how the new sending unit mounted on the transfer case. But from what I could find out, the gear inside the transfer case that is seen in the right photo above would have to be replaced, and that was something I did not have.

	So instead of using using factory way of picking up the speed, I mounted 8 very strong magnets to the drive shaft and mounted the sending unit to a angle bracket, and mounted the bracket to the cross member for the transmission.  Click on photo for larger image.
	I Later found a sending unit that if the gear inside the transfer case was good, or if I wanted to fix it, I could have used instead.   VDO part # 340-011  VDO also makes some nice programmable Speedometer with reset able trip odometers.

The new speedometer fitted in the old speedometer just fine.  Had to run a single twisted cable to the sending unit and got my power from the fuel & voltage gauge to power the speedometer. Set the dip switches in the speedometer to reflect the correct speed using a GPS unit.  Now I have a nice, smooth operating speedometer.
 
Old Speedometer

Speedometer Cable Replacement.