1997 Ford F-150 A/C

Nothing wrong. Most systems are spring-loaded to go to defrost when engine vacuum is lost. That's for safety. Your feet might freeze, but the windshield will be clear.

In this case you're losing intake manifold vacuum when you're accelerating or under load. First look for the vacuum hose going to the firewall that feeds the heater controls. That hose will be shown on the vacuum hose routing diagram on the sticker under the hood. Somewhere in that hose is going to be a check valve. That is usually about the size and shape of a stack of three nickels. Check that valve for proper operation. If you can blow through it both ways, replace it. If you can only blow through it one way, it's okay. Replace it with a check valve from a Chrysler dealer's parts department. Those have a storage canister built in to prevent this from happening. Those valves are about 1 1/2" in diameter and 2" long.