Morgan, there are a couple of things that could cause this. Most likely, it is the blend door (either the actuator or the door itself). The blend door diverts the air flow from the heater core (hot) to the A/C evaporator core (cold). If it gets stuck, broken or the actuator fails, you will have only HOT or COLD, depending on what position it gets stuck in. It could also be a defective HVAC control panel, but that is unlikely. I think your 2008 has the electric actuator, and those can burn out.
Now, based on your description "On regular air, it's either really cold(AC cold) or hot, no in between", it sounds to me like the blend door is broken. When this happens, the door just sort of flops around inside the duct, which could account for the "all or nothing" of hot or cold. You didn't state if you purchased the truck from a dealer or a private party. If from a dealer, you may want to take it back and have them fix it, as this is a bit of a chore to change. HTH