Since you have already swapped out the GEM and the axle sensor, you could try monitoring voltage on the gray/black. IIRC, my voltmeter shows about a 0~12v square pulse at very low speeds but this quickly becomes a midrange value as road speed goes up and the oscillation frequency gets too fast for the meter to track.
If you are not seeing any voltage on the gray/black when moving, try disconnecting the GY/BK from the speedometer, speed control servo and PCM. I'd do that one "user" at a time to see if one of them is dragging it down to zero.
Originally Posted by bclements515
So far I have continuity from the rear axle to the GEM and continuity from GEM to speedo.
Also, be sure that there is no continuity to ground on either of the two wires from the sensor to the GEM and RABS Module or on the gray/black from the GEM to speedometer, speed control and PCM. You would need to disconnect everything on each those wires to make the continuity-to-ground checks.