Why won't the car move if the air conditioner, heater, or defrost is turned on?

Are you old enough to know what a 'neutral drop' was? IF so , then you might get this , Mind you , this is coming from Honda experience ...not sure about GM software , if the PCM detects the 'throttle position sensor' reading is to high (whatever their pre-determined value for that is) it will not allow the trans to engage. For example if idle in gear the TPS is supposed to be .49V or less and it sees a value of .70V , it electronically prevents the trans from engaging to prevent damage. Hope that made sense , as to what the throttle assembly has to do with it. Good luck.