I have experienced this defect and the reason I say defect is because I have searched the internet and found others that have experienced the same issue.

I was assisting my mother-in-law into the vehicle rear passenger door (she ways probably about 190lbs) when the rear running board bracket broke. She is in her eighties and had I not had my hand on her back as I was supporting her entry she would have fallen back onto the sidewalk and my guess is probably hit her head which at her age would not have been good at all. When I got home and inspected my vehicle, I notice that the bracket had broken from what look to be metal fatigue because all four bolts were still attached to the frame with the metal under them but the remainder had broken free. I then when to the driver side and inspect that bracket and found two long cracks where the metal has already started to break away but it is still attached. When I researched it on line I found this area with similar complaints: http://forums.edmunds.com/discussion/8038/lincoln/navigator/lincoln-navigator-power-running-boards/p3

When I took it to the dealer they said they have to order the whole assembly at a cost of 1700+ dollars. Somethng is clearly wrong with this design and I would hope that ford is not slipping back into what drove customers away before, which was quality and reliability. I felt there "quality is job 1" slogan meant they were addressing the past and have since that makeover purchased two vehicles but will be looking to another brand if what I think is happening is happening.