HVAC problem.

If there is no physical damage, you can sometimes get it to work on many vehicles by pulling all the A/C and climate control fuses for about 20 min. When you put it back together and fire the vehicle up, it will re-index the blend door motors to the proper positions.........or unhook the battery and lose radio, etc.

I believe the plastic gears in the unit stripped out on mine causing the door to not operate. I could hear the door clunking trying to close.

Is it the blend door itself or the actuator when this happens?

My 2003 Chevrolet did the same thing, luckily for me it was the passenger side blend door not the one behind the dash. It only took 15 minutes to replace, if its the one behind the dash that'll cost you some money.

Thanks guys. Could it possibly be anything else?

Yep most likely your blend doors.

Could be expensive, but with a full size truck may not. It is probably the 'blend door' system in the HVAC unit, usually buried in the dash.
Had a similar problem with our 04 Accord, drivers side would always blow hot air 100degrees F, and passengers side would blow the cool air conditioned air, not pleasant in the summer. Talked to our trusted repair shop, 12-14 hours labour plus parts. They were working on a Dodge minivan, had a Ford Taurus waiting for similar problems. They have to remove the complete dashboard in all the cars to get to the blend system in the HVAC. I also checked with the Honda dealer, they said is not an uncommon problem, repair cost is $1495, we do them regularly.

Beverley and Ken