the door lock actuators are of a screw type, they tend to bind up after 10 years or so..
I've replaced both of mine..
should be ~$30 through ford.. simple to replace, too - pull the door panel off, locate the actuator, punch the center of the rivet out, drill out the rivet (1/4" drill bit) remove old actuator, install new actuator install new 1/4 inch rivet, shallow.. plug back in, and you should be set..