This package is priced at $4,940, and available for order now. That's a hefty price for what amounts to some minimal bolt-on modifications you could easily do yourself. However, buying it through GM means you get to keep your warranty in place.
The Sierra AT4 already has more off-road features than a normal Sierra 1500. You're getting a two-inch lift, four-wheel drive with two-speed transfer case, locking rear differentials, skid plates, Rancho off-road shocks and hill descent control. Both the wheels and tires are available as options already, but this package just bunches related stuff together. It'll be up to you to decide if an exhaust and intake are worth the extra dough.