Ive had the same problem. I have the 2005 Colorado I-5 though. You have to clean the throttle valve. It should eliminate the issue completely. I was getting the same shaking and such at around 500rpm with my Colorado and once I cleaned that its at a smooth 650-700prm. You are problably getting a Check Engine light with code P0506 (not sure correct). Which is low idle RPM issue. That was my case. When I cleaned mine, It didnt even look too dirty as I would of though, but it must be sticky enough to screw up the valve at low idle.
Post #298 covers this real good with the "how to" at the link below. Dont drop anything in inside the throttle valve! Use a big rag.
Only cost like 2-5 dollars to do this.