CAB air

Sounds like you have vacuum-operated heater controls and you're losing vacuum under certain condition, mainly acceleration when engine vacuum is normally low anyway. There will be a check valve in the small vacuum hose going through the firewall. If that is leaking it will not hold vacuum to the heater controls when you accelerate, most of those systems are spring-loaded to default to the defrost position for safety. Your feet might freeze but the windshield will be clear.

Check the check valve first. If it appears to be working Chrysler had an upgrade for this problem that may work on your truck. It is a larger check valve with a storage canister as part of it.