2003 GMC Sierra 1500 Pickup Owner's Manual - Page 468

Getting under a vehicle when jacked upis it is dangerous. If the vehicle slips the jack you off or could be badly injured killed. Never get under a vehicle when is supported only by jack. it a Raising your vehicle with the jack improperly positioned can damage the vehicle and even make the vehicle fall. To help avoid personal injury and vehicledamage, be sure to fit the jack lift head into the proper location before raising the vehicle. Front Position 2WD 1500 Series 5-93