Could be an open or short in the speed sensor circuit or erratic signal.
The strength of the signal can be affected by resistance in the sensor,
resistance in the wiring and connectors, metallic debris on the end of the sensor, and the air gap between the sensor and tone ring.
Are you getting the generated signal from the VSS ?
If not then is it possible the tone ring is damaged or is the air gap is to far to generate it ? Worst case > Transfer case wear issue...
Also check the wiring/ weatherpak plugs to and from for corrosion to the EBCM and from / to the PCM ?
Worst case > Could be a bad EBCM.
A good wheel speed sensor will generally produce an alternating current (AC) voltage reading of 50 to 700 MV when the wheel is spun by hand at about one revolution per second. Refer to the shop manual for the sensor's exact voltage specifications.
The location of the EBCM was a bad location designed by the GM engineers.