It has to be the tires! I had a 2wd extended cab silverado with Michelin ltx m/s tires on it. I live in Michigan and frequently drive unplowed dirt roads. In the 10 years I had the truck the only issue I ever had was making it up a hill on a unplowed dirt road that had 10" of fresh snow and about 7" in snow packed down on it. I had to make a few running starts to make it up the hill. It was a long (about 1/2 mile) steep incline. This was with no weight added to the bed. My 05 yukon xl has the same tires. The thing never slides sideways unless I want it to. I only bother putting it in 4x4 if I have more than 6" of fresh snow. 4x4 is just a a necessity in my xl. My dad had those coopers on his 2wd Avalanche and said that they were horrible in snow compared to the michelins he replaced