GMC 1996 Sierra Owner's Manual page 212

Parking on Hills
You really should not park your vehicle, with a trailer
attached, on
a
hill. If something goes wrong, your rig
could start to move. People can be injured, and both
your vehicle and the trailer can be damaged.
But if you ever have to park your rig on a hill, here's
how to do it:
1.
2.
3.
4.
5.
6.
Apply your regular brakes, but don't shift into
PARK (P) yet for an automatic transmission, or into
gear for
a
manual transmission.
Have someone place chocks under the trailer wheels.
When the wheel chocks are in place, release the
regular brakes until the chocks absorb the load.
Reapply the regular brakes. Then apply your
parking brake and shift into PARK (P) for an
automatic transmission, or REVERSE (R) for a
manual transmission.
If you have a four-wheel-drive vehicle, be sure
the transfer case is in a drive gear and not in
NEUTRAL (N).
Release the regular brakes.
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.
If you have left the engine running, the vehicle
can move suddenly. You or others could be
injured. To be sure your vehicle won't move, even
when you're on fairly level ground, use the steps
that follow.
If you have four-wheel drive and your transfer
case is in NEUTRAL (N), your vehicle will be free
to roll, even if your shift lever is in PARK (P). So,
be sure the transfer case is in a drive gear
--
not
in NEUTRAL (N).