Mercury 1999 Mountaineer Owner's Manual page 180

Roadside emergencies
When one of the rear wheels is off the ground, the transmission
alone will not prevent the vehicle from moving or slipping off the
jack, even if the transmission is in P (Park).
3. Block the diagonally opposite
wheel.
4. Use the tip of the lug wrench to
remove any wheel trim.
5. Loosen each wheel lug nut, but
do not remove them until the wheel
is raised off the ground.
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