When in gear and ac is on my van will stall out if stopped?

Check the electrical connector at the "IAC" on the cold air intake adjacent to the throttle body. 3 wires, and with key in "run" position, one should be grounded, one should show battery voltage (12 VDC) and the other should show "Reference voltage" (4.6-5.8 VDC)If the wire's values match up, then the IAC itself is probably inop. See if it makes any difference in idle quality if the IAC is unplugged. No difference means IAC is not working. Other than IAC concerns, the one main cause of this concern would be the Torque Convertor Lock-up Solenoid is stuck, which will make the car act the same as a manual transmission would act if it were stopped without pushing in the clutch.