Is the truck the new body style or the classic body style??
Does the passengers side blow hot or cold when this happens??
The a/c temp doors are controled by eletrical actuators, They do go bad, when they do, the a/c controller will cammand them to full hot. This is a common concern on this body style truck. Normally when this happens, it happens to the drivers or the passengers side vents, not both at the same time.
Let me know what happens.