I think there are two different functions that are mixed up here:

a) Auto-locking/-unlocking when driving off and coming to a stop. This one can be turned on/off through PCM if you have the Sport Chrono (or through the dealer if you don't).

b) The auto-re-locking when you unlock the car but don't open any door within a certain period of time. I believe this is a standard feature which can't be disabled. However, it also reduces the problem of unlocking the car unintenionally with the key in your pocket. Unfortunately, that doesn't work with the trunk.