If you have a late model Ford truck you can enable or disable the auto-lock feature by follow the directions below. Generally the auto-lock feature is turned on by the factory when the truck has an automatic transmission and it is turned off from the factory on trucks with a manual transmission.

Start in the truck with all doors closed and the key turned off.
Put the switch in the on position and press the door unlock button 3 times.
Put the switch in the off position and press the door unlock button 3 times.
Put the switch in the on position, you should hear a quick note from your horn.
Press the door unlock button 1 time then press the door lock button 1 time.
You should hear a short note from your horn indicating that the auto-lock system is disabled. If you hear a short note then a longer note from the horn then the auto-lock system has just been enabled.
Turn the switch to the off position. You will hear a short note from the horn to let you know that the system has been changed.
These steps should be done within 30 seconds in order to properly activate or deactivate the feature.

The system will in most cases revert back to the original setting anytime you disconnect the battery. I understand that these directions will work on any late model Ford vehicle but I have only performed it on the Ford trucks.