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: 101,783 mi (35,000 mi - 160,000 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.
Visitor, 2000 Chevrolet 3500 Pickup, 160,000 mi
door lock not working
Have had to replace the lock actuator on the rear passenger twice already.