I've been frustrating myself dealing with an idle issue and am hoping I'm missing something as to the cause (and remedy).

As the car is right now, if I turn it on, no-load idle will be around 1500rpms. When I put it into gear (drive or reverse, both show the same symptoms) it drops to approximately 750 rpms.

I can adjust the idle screw to bring the no-load into proper range (7-800) but then it stalls out in gear.

Other things I've done in an attempt to fix this:

Replace the Idle air motor.
Check for vacuum leaks.
Play with kickdown and throttle lines tightening/loosening them.

Additionally, I feel there is a larger than normal rattle in the rear exhaust (as though engine/tranny vibrations are transferred to it causing the noise) while stopped in gear. This could just be that the over-the-axle pipe I installed is able to tap and it wasn't before.

Other recent (last 2000 miles) repairs I've done:

New exhaust headers back.
Motor/tranny mounts
Cap/rotor/plugs


Any ideas where to go next to solve this?
--
If you're not driving it "like its stolen," are you really driving?