The power door lock actuators may become slow to operate or stop working completely and require replacement.Google+
Car Problem Reports
Chevrolet 3500 Pickup Power Door Lock Actuators May Fail
Chevrolet 3500 Pickup Problem
Average Mileage: 106,066 mi (85,000 mi - 127,132 mi)
Yeap, my 2000 Chevy C3500 truck is doing the same thing at the left rear passenger door door lock. Basically mine is not locking or unlocking and I know there is power to the door lock actuator but the door lock actuator is too weak to move the door locks to lock/unlock. I just ordered a door lock actuator off of Ebay and will replace it when it comes in.
Have had to replace the lock actuator on the rear passenger twice already.