2010 GMC Sierra 1500 Crew Cab Owner's Manual - Page 169

P (Park): This position locks the rear wheels. It is the best position to use when starting the engine because the vehicle cannot move easily. When parked on a hill, especially when the vehicle has a heavy load, you might notice an increase in the effort to shift out of P (Park). See Torque Lock (Automatic Transmission) under Shifting Into Park on page 3‑55 for more information. { WARNING: If you have Four-Wheel Drive, the vehicle will be free to roll - even if the shift lever is in P (Park) - if the transfer case is in Neutral. So, be sure the transfer case is in a drive gear, Two-Wheel Drive High or Four-Wheel Drive High or Four-Wheel Drive Low - not in Neutral. See Shifting Into Park on page 3‑55. R (Reverse): Use this gear to back up. Notice: Shifting to R (Reverse) while the vehicle is moving forward could damage the transmission. The repairs would not be covered by the vehicle warranty. Shift to R (Reverse) only after the vehicle is stopped. To rock the vehicle back and forth to get out of snow, ice, or sand without damaging the transmission, see If Your Vehicle is Stuck in Sand, Mud, Ice, or Snow on page 5‑31. { WARNING: It is dangerous to get out of the vehicle if the shift lever is not fully in P (Park) with the parking brake firmly set. The vehicle can roll. Do not leave the vehicle when the engine is running unless you have to. If you have left the engine running, the vehicle can move suddenly. You or others could be injured. To be sure the vehicle will not move, even when you are on fairly level ground, always set the parking brake and move the shift lever to P (Park). See Shifting Into Park on page 3‑55. If you are pulling a trailer, see Towing a Trailer on page 5‑55. 3-31