My brother has a 2002 F-250 4x4 7.3 extended cab. Today he was stuck and had to lean on it pretty hard to get out. Now the defrost / A/C vent won't switch between them. He said he had this problem before and the actuator was out after a similar event.

Found the answer on another post. Sorry for repeating it!

 

If your truck has a vacuum leak, the heating system will only blow out of the defrosters.