Great web site! I'm amazed the information that you folks have and how you freely share troubleshooting tips. On my wifes 2003 Yukon XL, it always feels that you are in 4WD. I doesn't matter if you are on dry, wet pavement or ice. The light will even go into the 4WD Hi position by itself. The only time when it dosen't feel like we are in 4 Hi is when I have it in 2 WD and then every know and then it will also go into 4WD. I suspect the ATC but welcome any suggestions.