Judging by the error code I would be willing to bet it's a 2004 w/o cladding. I had the same problem in my 2004 a couple years ago. For some reason the problem seems to be persistent in the '04 models. I tried replacing the gas cap but still had problems with the CEL and hesitation when filling up. Another member pointed me in the direction of the vent valve and evap canister http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,107546.msg1693466.html#msg1693466
which solved the problem and never returned again after replacing those parts.
Here is the thread on actually replacing the parts if you choose to do so, including diagrams and part numbers. http://www.chevyavalanchefanclub.com/cafcna/index.php/topic,88017.0.html
Good luck and hope you get it figured out!