Being in IL, I assume you have the Chevy 229ci 3.8L V6 (not the Buick 3.8L)?
The '80 229 doesn't have the idle control of the later computer models. Just a solenoid to raise the idle a bit with the compressor on.
You should check the a/c solenoid as 80ECLT said, could be stuck or something.
After that, see if the throttle stop is against the curb idle adjustment screw. If it is not touching the screw, then the high idle cam is likely sticking and holding the throttle open.
Also possible you have a vacuum leak somewhere, that will raise the idle up. Or could be that someone before has turned the idle screw to raise it on purpose. The 229 was known for an idle vibration, and raising it up may have been someone's "cure." Should have ~750 rpm in neutral, 600 in drive.