Just bought a 2003 Yukon XL 2500 with the 8.1 liter. I live in Montana and am going to tow a camper with it. My problem is the four wheel drive only engages in the four wheel auto and two wheel driver positions. When I try to engage four hi or four low I get now power to the front axle. I can hear the axle engage but get no power to it. I haven't yet lifted the truck to see if the the front shaft is spinning or to check the axle is locking. Have a foot of snow on the ground and don't feel like getting soaked just yet. The lights on the selector switch show it's in four hi and low. just wondering if anyone has any ideas.