The 03+ are open like a one leg rear axle. They modulate torque front and rear by using the ABS and Stabilitrac, not mechanically.
Why no AWD?
5.3L, 4L60E, True Dual Exhaust, 40 Series Flowmasters, K&N Cold Air Intake, Nelson Performance PCM, 93 Octane, 0% TM, Ram Air Hood, 35" Pro Comp X-Terrain's, 18" Mazzi Sphinx Wheels. 3000 stall Yank, TCI clutches, Trans-go Shift Kit. Soon To Come:
4.56's, Cam, PLT's
If you really want an AWD case, get the NP149. It is what the C3 and early Denalis had. It isn't open.
^^^ +1