Some of the information you didn't give was where and how you mounted the antenna. Further, you didn't state whether you have bonded the various pieces of the truck to the frame, such as the doors, hood, and cargo box. All of these are important parts of preventing ingress of RFI to the EEC. Visit my web site for more information.
Another problem ifs the size of wire you used (?) and how you ran it to the battery. Running the power cable next to existing wire, and impressing RFI on to the coax due to a poor antenna installation is a sure way to get frustrated.
Lastly, GM products are no worse for RFI ingress than any of the others out there. Nonetheless, they (GM) publish a rather good treatise on RFI prevention. The URL has been published here on this forum several times. Unfortunately, I don't remember what the URL is.