2003 GMC Sierra 1500 Pickup Owner's Manual - Page 138

It can be dangerousto get out of your vehicle if the shift lever is not fully in PARK (P) with the parking brake firmly set. Your vehicle can roll. Don't leave your vehicle when the engine is running unless you haveto. If you've left the engine running, the vehicle can move suddenly. You or others could be injured. To be sure your vehicle won't move, even when you're on fairly level ground, always set your parking brake and move the shift lever to PARK (P). Four-wheel drive vehicles with the transfer case in NEUTRAL will allow the vehicle to roll, even if your shift lever is in PARK (P). So, be in sure the transfer case is a drive gear - not in NEUTRAL. Always set your parking brake. Follow the proper steps to be sure your vehicle won't move. See Shifting Into Park (P) on page 2-45. J 2-5 1 If you're pulling a trailer, see Towing a Trailer on page 4-66.