I've seen quite a few threads asking about idle problems on these trucks. 99% of the time the problem is the IAC. The Idle Air Control is a valve that bypasses the throttle body and allows the computer to control idle speed. This valve can become clogged with carbon and such and stop working. You can either try to clean it out with throttle body cleaner and a toothbrush or you can just replace the valve. It is around $40 at the parts store. The valve is located on the intake plenum behind the throttle body. Its a bit of a tight fit but can be done. Replace the valve and your idle problems should go away.