more than likely, you have a bad actuator in either of the front doors, more than likely the drivers door. There is a way to fix it without replacing the actuator, but you have to be good at prying apart the actuator that is glued together without breaking it and then you have to take the gears out and snip a wire or replace the little rubber stops that have over the years broken down. In a nut shell, there are little contacts inside the actuator that are used in the 'central locking system' Its complicated, I fixed it in one of my monteros, but I would recommend just getting a driver side door actuator from a part yard for 40 bucks and replacing it, its fairly easy. when the rubber contacts break apart, it lets the control gear (shaped like a pizza slice) travel past its normal range of motion and in turn the electrical contacts touch in a way they weren't designed to, so what is happening is when you unlock the truck, the actuator over extends itself and sends a signal to the central locking mechanism to lock the doors even though you never intended it to happen. the complicated fix is to open the actuator and either repair the rubber stops somehow or snip the green wire that is responsible for the central locking, your remote will still work, but opening the door with the key will no longer open all doors nor will the key lock all doors either. if I confused you, let me know, I will go into more detail. I did a long write up on this years back, don't know where it went though.
HTH
scottie
