Rear Suspension Shock Absorber problems of the 2012 Ford F-350

Death wobble: driving over a rough patch of freeway or over an overpass cross joint between 50 - 65 miles per hour, I experience violent shaking due to oscillation of the front wheels and axel. Must reduce speed to ~30 miles per hour to make the shaking stop. I have replaced/upgraded front shock absorbers and steering damper with little effect. I will next replace tires, align the front end and have the track bar (panhard bar) replaced. This first happened in August 2015, the dealer told me it was normal. My truck is now out of warranty, and this happens regularly. I know a spot that the oscillation will occur, and drive to avoid it, however it happens regularly without warning.