Sounds like your blend door actuator is sticking. The use an electric motor to do the switching. It is almost all plastic and the gears tend to break with age. You have to remove a panel under the glove box to get to it.
so if the heat returned to the off position after shutting down the truck and walking away.....
sounds like the door moves. if it moves, the gears must be ok.
Yeah but it can get stuck closed on the cold side and only allow hot air. Mine is messed up on my driver side in my 04 since it has two blend doors one for driver and one for passenger. I can be driving down the road with the a/c on and the driver side blend door will shut the cold side off and I'll be getting crazy hot air while the passenger still has nice cold air. The only way for me to reset it is to pull over and shut the truck off and start it back up and everything is fine again. This will not affect the ability turn the heat off though you just can't control the temperature.