I had a similar fault on my Sierra and took the fuse out feeding the central locking system which disabled it and was able to lock and unlock the drivers,passengers and boot locks manually .
My fault turned out to be the solinoid on the drivers door.
Unless you've already done it.
Once in the car drop the fuse board and remove the fuse feeding the central locking system. OPEN the drivers window [to make re-entry easier if the following doesn't work.] Open the door [the lock leaver should jump up as you do.] Then try locking the door, then unlock it. If it doesn't work try the front passenger lock it that works it could be what Mike says the barrel is worn. Replace the barrel then the fuse operating the central locking then see what happens but don't forget to leave a window open for easy access. If the lock works but not the central locking mode then it could be a solinoid on the drivers door lock or even the locking system relay.
bob
bob
January 2005