2000 F250SD electric door locks

My door locks work poorly whether using the button on the door or the keyless entry. When the locks do work the back doors never unlock/lock they look/sound like they're trying to work but dont, if they do its just once every other day. The batteries are good fuses too. Please advise.
Andy

Sounds like the door lock motors are worn out. Just to be sure you can take off the door panels and inspect the motor. If you see any cracks or anything it is probably worn. Also you can try taking the motor out and give it 12 volts and if it makes the same noise and doesn't move then you need to replace it.

I have the same problem axcept the driver side is still working. I've be told that maybe a relay is bad. If each door has a seperate relay, that could be the problem if not enough juice is getting through the relay. I don't know were the relays are so I can't test them. I am going to buy a shop manual on this truck. Have you found the problem with yours yet?