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I had the same issue. It's the door lock actuators. They need to be replaced. It's a PITA the first time to pull the door off and replace an actuator. It's easier on the second one.
Here is some info from an old post of mine (note: the place I got mine from are apparently gone, the URL is invalid):
I just ordered the actuators from a friend from FORD, I got them for $49 a piece. You can get them from http://www.genuine-ford-parts.com/ for $46 plus shipping. The local Ford Parts wanted $86 a piece.
The part numbers are 6L3Z25218A42AA for passenger side and 6L3Z25218A43AA for drivers side.