The power door lock actuators may become slow to operate or stop working completely and require replacement.Google+
Car Problem Reports
GMC 1500 Pickup Power Door Lock Actuators May Fail
GMC 1500 Pickup Problem
Average Mileage: 126,000 mi (88,000 mi - 150,000 mi)
I have no idea how to fix it, but the manual door lock will not lock.
Visitor, 1994 GMC 1500 Pickup, 140,000 mi
Check voltage to actuator if lower than 12 volts replace switch.If 12 volts check for binding in linkage in lock assembly,if not replace door lock actuator.
Visitor, 1998 GMC 1500 Pickup, 88,000 mi
door lock switch in door panel not working