Chevrolet 2008 Silverado 1500 Pickup Owner's Manual page 149

CAUTION:
{
It can be dangerous to 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. Do not leave your 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 your vehicle will not move, even
when you are on fairly level ground, always
set the parking brake and move the shift lever
to PARK (P).
CAUTION:
{
Four-wheel drive vehicles with the transfer
case in NEUTRAL will allow the vehicle to roll,
even if the shift lever is in PARK (P). So, be
sure the transfer case is in a drive gear — not
in NEUTRAL. Always set the parking brake.
Follow the proper steps to be sure your vehicle will not
move. See Shifting Into Park on page 2-51.
If you are pulling a trailer, see Towing a Trailer on
page 4-55.
2-55