One problem related to power train driveline has been reported for the 1999 Ford F-150. The most recently reported issues are listed below.
My son was driving his 99 f150 southbound on a two lane highway, going about 50 mph when he heard a pop, and then the truck went out of control. The rear of the pickup began to travel left across the highway. In the northbound lane, it began to roll over, drivers side first, finally landing upright at the left edge of the highway, then slid down an embankment front first in some trees. The left rear axle had fractured and snapped and the hub and wheel had come off. Thankfully, he and his girlfriend suffered no permanent injuries.