Mercury 1999 Mountaineer - Manual (Page 180)

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.
Roadside emergencies
180