When I turn on my A/c or vent, it barely blows out the dash ducts but blows pretty well out of the floor vents. I know the actuators are vacuum operated, but which one controls where the air goes? How do you diagnose them?

 

Floor/defrost (somewhere between the two) is the default, no vacuum, position for the doors. It sounds like you have lost vacuum to the HVAC panel in the truck. I would start looking there.