The AC/heater (HVAC) system may unexpectedly switch to the defrost mode when accelerating. This system is operated by engine vacuum and should be inspected for any vacuum leaks if this problem develops. There is also a revised vacuum check valve available to address this concern.Google+
Car Problem Reports
Dodge Ram Wagon 3500 HVAC Air Flow Switching to Defrost When Accelerating
Dodge Ram Wagon 3500 Problem
Average Mileage: 163,000 mi (120,000 mi - 206,000 mi)
Visitor, 1995 Dodge Ram Wagon 3500, 206,000 mi
Based on how fast the van is going the air from the vents switches to defrost and heat vents.