I just picked up my 3LT 3.43 axle this weekend, and it only has 100 miles on it or so, so I'm not sweating my 12.6 avg mpg - it will get better once the engine is broken in more. I can tell you that both 4WD and the 3.73 axle will cost you about 1 mpg apiece, according to my dealer. YMMV, of course. 
I can also tell you that we rented a Burb this summer for a roadie to Arizona, and the Burb (which was a 07 LT w/4WD), fully loaded with 6 passengers + luggage and me driving like a bat out of hell got 18 mpg round trip, which I considered pretty outstanding considering the conditions it was under.
I can also tell you that we rented a Burb this summer for a roadie to Arizona, and the Burb (which was a 07 LT w/4WD), fully loaded with 6 passengers + luggage and me driving like a bat out of hell got 18 mpg round trip, which I considered pretty outstanding considering the conditions it was under.
