It does only come with the 5.3L V8.
(There is mention of the 6.0L on the website on the 'Trailering Tab' but no apparent option or way to build & price it. Perhaps special order/consult your dealer.) The 5.3 w/ the 4.10 is plenty peppy, and it tows fair. HOWEVER, the truck does have coil-rear springs, so there is no way I would tow more than 4000 or so lbs. without rear air-bag suspension.
There is no more 2500 model.
3.73 rear axle is standard.
4.10 rear axle is optional.
These are the mileage numbers off of Chevy.com:
2WD (gasoline) 15/21
2WD FlexFuel (E85) 11/15
4x4 (gasoline) 15/20
4x4 FlexFuel (E85) 12/16
www.edmunds.com usually has fuel mileage numbers that are dead on.
Its a nice truck. I used to own a 2500 w/ the 8.1L, and it towed 12,000ish lbs. adequately. I'd buy another current model if it was feasible as a daily driver.