2000 Escalade. My front doors lock, and my rear passenger doors lock, but my rear cargo doors DO NOT lock. The lock rocker switch on the pillar by the cargo doors activates the front four doors but nothing on the cargo doors.
With a multimeter, look for 12vdc on the cargo door lock solenoid when the LOCK switch is depressed. Your symptoms point toward a faulty solenoid. The mechanism might be binding. It could be easier to access the connector that supplies the voltage, depending on location. A GM shop manual is a solid investment and cheaper than two hours of shop labor. With it you can quickly locate the best method to access the solenoid or determine where the supply voltage is located in the connector.