My 2003 Expedition has that problem of changing air flow from panels to the floor and defrost vents when the engine is under a load. As soon as I let off the gas, the air flow changes back to panel from defrost but when I accelerate again, it changes to back to panel. I read a little about it being vaccuum hose, vacuum motor but couldn't get a definitive answer.

 

My 2004 has the exact same problem. Started about a week ago. I have heard thi sis a vacum leak, but I don't know. Can someone with this knowledge plaese help. THANKS in advance.