In August of 2009, the National Highway Traffic Safety Administration launched an investigation of the 2002-2005 Ford Explorer, the 2003-2005 Lincoln Aviator, and the 2002-2005 Mercury Mountaineer. According to a notice on NHTSA's website, the agency had received numerous reports of those SUVs mysteriously slipping out of park.
During the course of the probe, NHTSA examined 180 related complaints, many of which involved vehicles rolling away from the spot where they were parked. Some problems were traced back to the parking system itself, others were related to the brake-shift interlock (i.e. the safety device that prevents drivers from putting a vehicle in "drive" unless the brake is depressed). The Detroit News estimates the total number of vehicles involved in the probe at 1.56 million.
But that's all ancient history because NHTSA has called off the investigation. However, the agency was quick to point out that "The closing of this investigation does not constitute a finding by NHTSA that a safety-related defect does not exist". Instead, NHTSA nixed the probe simply because "there has been a drastically declining trend including very few incidents occurring in recent years". In other words, no one's really complaining about this anymore, so it's not much of a problem.
Although NHTSA doesn't come out and say it, part of the rationale for halting this investigation may be because some of the reported "problems" with Ford's SUVs appear to have been caused by "user error". In a letter dated December 2, 2011 [PDF], a Ford representative states that:
Ford agrees with [NHTSA] that drivers who do not follow basic, common sense vehicle driving practices that are consistent with safe vehicle operation may experience unintended vehicle movement while in Park. These practices include: ignoring any progressive changes to the shift system behavior or feel, ignoring the fact that the shift indicator is not in the Park position, ignoring the fact that the key could not be removed from the ignition switch, and/or ignoring the Owner's Guide instructions and the agency's recommendations to always apply the parking brake before exiting the vehicle.
Translation: if drivers would use a little common sense and stop being such litigious whiners, this wouldn't be a problem.
Click here to see NHTSA's official statement about ending the probe and to find links to 20 other documents related to the investigation.