FORD 2003 F-150 v.3 - Manual (Page 188)

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) or in the reverse gear
(manual transmission). To prevent the vehicle from moving when you
change the tire, be sure that the parking brake is set and the
diagonally opposite wheel is blocked. If the vehicle slips off the jack,
someone could be seriously injured.
• Front (4x2)
• Front (4x4)
Roadside Emergencies
188