My 05 Yukon XL with 288k has got a 4wd issue. Just bought the truck about three months ago, and never tested the 4wd system prior to buying. The lights on the buttons illuminate, but shifting to neutral and attempting to shift into 4hi, 4lo, 4auto, or even 2wd yields no feedback. In fact, the buttons don't even show what the current configuration is. I am pretty sure it's in 2wd based off driving behavior but I could be wrong. Having looked at plenty of forum responses, everyone says that when this happens, they get a "service 4wd" messages. I have no messages, nothing that indicates the system is malfunctioning.