2005 GMC Sierra 1500 Pickup Owner's Manual - Page 127

While you accelerate, it is normal for the light to go on and off if you quickly change the position of the accelerator. If your vehicle has four-wheel drive and is equipped with a manual transmission, disregard the SHIFT light when the transfer case is in 4-Wheel Low. Ignore the SHIFT light when you downshift. Front Axle Locking Feature The front axle locks and unlocks automatically when you shift the transfer case. Some delay for the axle to lock or unlock is normal. Manual Transfer Case Four-Wheel Drive If your vehicle has four-wheel drive, you can send your engine's driving power to all four wheels for extra traction. To get the most satisfaction out of four-wheel drive, you must be familiar with its operation. Read the part that follows before using four-wheel drive. See the appropriate text for the transfer case in your vehicle. Notice: Driving on clean, dry pavement in four-wheel drive for an extended period of time may cause premature wear on your vehicle's powertrain. Do not drive on clean, dry pavement in four-wheel drive on pavement for extended periods of time. The transfer case shift lever is on the floor to the right of the driver. Use this lever to shift into and out of four-wheel drive. 2-37