Why does the a/c in my 2003 expidition blow out of the defrost instead of the vents when accelerating over 40 mph ?

You have a vacuum leak.Defrost is the default position (no vacuum).
You have a vacuum reservoir that mounts behind the battery. This is for the reserve vacuum to the climate control while you are accelerating,since while accelerating you loose manifold vacuum.
You will see a vacuum line that goes from here to the intake manifold then one that goes from here to the fire wall near the heater coolant hoses and into the car to the controls. Check this carefully and you should find a leak some where in the lines or the reservoir.