LOL LOL Man have I been all thru this when I bought my 2005 XL/SLT.
In a nutshell: It's a rare (Stabilitrac) option on a Yukon. What you (I) have is a hybrid cross between AWD (Denali) and 4x4 (Suburban). Denali's are AWD all-the-time = the worst gas mileage. Suburbans and most Yukons have 4-H/4-L/2-H. We have a computer-controlled system that directs power to computer-determined wheels WHEN NEEDED...AND have a 4-L option button. Just drive as normal and you let the computer decide when to more-or-less engage 4x4 . It also has the traction control. I TRIED to skid around and do donuts on an ice covered lot...it wouldn't! Gas mileage isn't bad = 21/22 mpg highway on mine. I could go into more detail, but that's basically it.