2003 Chevy Venture Minivan --Heater only heats in Drive?

I have a 2003 Chevy Venture minivan, 67k. Never any problems until last week. Now when I turn on the heater it blows in all the positions (including defrost), but only blows COLD air while the van is in park. This is regardless of how long I let the engine warm up. Once I shift into drive, the warm air starts to blow very quickly. It switches back to cold air when I go back to park. What could be the problem here? Thank you!