I have a 94 GMC Sierra 1500, 350 5.7 L.

If you start it up it runs around 700 RPM in park, about 400 if you put it in gear. Same with the AC on. Once it warms up it only runs about 400 RPM in park, 100-200 in gear. With the AC on, when you come to a stop its shaking pretty bad.

I've had it to two shops. Both verified AC compressor is fine. I have put in new spark plugs, wires, and dist. cap. No improvement

No computer errors, not running rich. ONce in a while if you are stopped with the AC on it dies.

Anyone have any ideas?

check your egr system for leaks and or malfuctions

My bet would be the IAC control, passenger rear of tbi,
this is the item my haynes says to check