GMC 2001 Sierra - Manual (Page 331)

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2. Position the jack under the vehicle. If the flat tire
is on the front of the vehicle (2 - Wheel Drive
1500 Series vehicles), position the jack under the
bracket attached to the vehicle’s frame, behind the
flat tire. If the flat tire is on the front of the vehicle
(all other models), position the jack on the frame
behind the flat tire.
If the flat tire is on the rear, position the jack
under the rear axle between the spring anchor and
the shock absorber bracket.
If you have added a snow plow to the front of
your vehicle, lower the snow plow fully before
raising the vehicle.
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.