Looks like you have a 3.73 rear end. If you want more info on your truck while waiting for the build sheet from the gm reps go to:
http://chevroletforum.com/forum/vindecoder.php and type in your VIN.
Engine, Vortec 6.0L Variable Valve Timing V8 SFI (360 hp [268.4 kW] @ 5400 rpm, 380 lb-ft of torque [513.0 N-m] @ 4200 rpm) with (E63) fleetside pickup box, 312 hp [232.6 kW] @ 4400 rpm, 373 lb-ft. [503.5 N-m] @ 4400 rpm with (ZW9) pickup box delete)
Rear axle, 3.73 ratio
Pickup box, Fleetside
Air cleaner, high-capacity
Cooling, auxiliary external transmission oil cooler heavy-duty air-to-oil
Cooling, external engine oil cooler heavy-duty air-to-oil, integral to driver side of radiator tank
Transfer case, electronic shift with rotary dial controls (Requires 4WD models)
Four wheel drive
Battery, heavy-duty 600 cold-cranking amps, maintenance-free with rundown protection and retained accessory power
Alternator, 125 amps
Frame, hydroformed front section
Recovery hooks, front, frame-mounted
GVWR, 9200 lbs. (4173 kg)
Suspension, front independent, torsion bar
Suspension, rear 2-stage multi-leaf springs semi-elliptic
Steering, power, recirculating ball
Brakes, 4-wheel antilock, 4-wheel disc
Exhaust aluminized stainless-steel muffler and tailpipe