Here is a two page post someone made showing how to remove the upper intake which you have to do to work on the IMRC. On page 2, step 46 is another link which will take you to another write up showing the IMRC and what needs to be done to fix them.
The BEST write up and pictures I've found showing how to do them!
My 2003 4.2 threw the code last year for the first time. I ran Sea Foam through the engine, disconnected the battery, and the light stayed off for over a year. Just last week it came back on. I disconnected the battery, and again so far the light has stayed off. I am just putting off the inevitable, as I know I will have to repair these sooner or later. But I am not going to pay $800.00 at the dealership to get this done! That's why I bookmarked that write up last year.