Could be several different things but I would clean the throttle body first if it hasn't been done recently. If it's never been done, it definitely needs it. They get pretty nasty on these trucks pretty quick. You can also clean your idle air control valve and throttle position sensor while you have the TB off. Just be careful with the sensors as they are easy to damage. If you do a little searching you will find gobs of info, pics, and probably vids on removal and cleaning of these pieces.
Good luck and hope this helps!
Good luck and hope this helps!
