ac/heater

The air switching mode doors on your Ram 1500 are most likely vacuum operated. When you accelerate engine vacuum is being lost and the mode doors move the their default position which is the defrost mode. You should begin by checking the vacuum lines under the hood as they relate the the a/c system. You should find a vacuum supply hose, a one way vacuum check valve, a vacuum reservoir, and a vacuum hose the runs through the fire wall to the vacuum switching valve behind the dash. Check all vacuum components under the hood for leaks, paying special attention to the check valve and the reservoir - those are the two items the play the largest roll in keeping vacuum supplied to the switching valve when engine vacuum is reduced.