No one has mentioned the axel ratio, I guessing since the post was about a 4 x 4. Mine is a 09 2WD Yukon XL 1500 and has the 3.08 axel ratio. I get 21 on the hwy -18 in the city, 11 towing our small camper. The trade off for that ratio is that the towing capacity is 5400 (1833 payload) and their is no towing package. I added a trans cooler just to be safe.
You can check the RPO codes on the sticker in your glove box to determine your ratio.
GT4 AXLE REAR,3.73 RATIO (DUP WITH 5X1)
GT5 AXLE REAR,4.10 RATIO (DUP WITH GTS)
GT7 AXLE REAR,3.33 RATIO
GT8 AXLE REAR,4.10 RATIO (DUP WITH GT5)
GU2 AXLE REAR,2.73 RATIO
GU4 AXLE REAR,3.08 RATIO
GU5 AXLE REAR,3.23 RATIO
GU6 AXLE REAR,3.42 RATIO
Also, code G80 indicates the factory rear locker.