2005 Explorer - Air Conditioning Question

I've got a 2005 Explorer - 4.0 6cyl and AT. With one exception, the A/C works fine. At interstate speed with cruise control on, the airflow will divert from the dashboard vents to the defroster vents when the vehicle accelerates, such as climbing a hill. After I peak the hill, the airflow returns to the dashboard vents. On level stretches of road, or at slower speeds, everthing is fine. Any thoughts or suggestions? Vacuum leak, perhaps? If so, how do I begin to find out where the leak is?? Thanks very much for your help, ~OC~