GMC Sierra 2008 Owner's Manual page 494

Make sure that the jack head is positioned so that
the rear axle is resting securely between the
grooves that are on the jack head.
CAUTION:
{
Getting under a vehicle when it is jacked up
is dangerous. If the vehicle slips off the jack,
you could be badly injured or killed. Never
get under a vehicle when it is supported
only by a jack.
CAUTION:
{
Raising your vehicle with the jack improperly
positioned can damage the vehicle and even
make the vehicle fall. To help avoid personal
injury and vehicle damage, be sure to fit
the jack lift head into the proper location
before raising the vehicle.
5-102
5. Turn the wheel wrench clockwise to raise the
vehicle. Raise the vehicle far enough off the
ground so there is enough room for the spare
tire to fit under the wheel well.
6. Remove all the
wheel nuts and
take off the flat tire.