sfc1963, from the sounds of things, your latch mechanism is binding up some and it is pulling the lock shut as the latch is closing. I would first try spraying a bunch of WD-40 into the latch and then see what you end up with. May end up replacing the latch.
This all stems from the fact that Ford latches have a feature built into them that when the locks are locked and you are inside of the truck, when you pull on the door handle the doors will automatically unlock. In your case, this feature is sticking and as the latch closes, it pulls the lock down.