2002 ford expedition max ac only blows out thru defrost vents?
Check for vacuum leak--either under dash or in engine compartment Check for bad vacuum motors under dash Defrost is the default position for this system There is a vacuum resovoir located behind your battery , it is for this condition.
When you accel your engine doesn't produce vacuum due to the throttle plate being open so a resovoir is there to hold a vacuum reserve so that when there is low vacuum produced by the engine there will be reserve to hold the vacuum motor applied preventing the air from changing direction in your dash.
When there is no vacuum the direction is at the defrost vents if you have a broken hose or no vacuum your ac would blow out the defrost all the time
But since it happens when you accel it sounds like your vacuum resovoir is cracked thus not allowing it to hold a vacuum