Front-axle death wobblemajor
The Super Duty uses a solid front axle, and when a front-end component wears, the axle can start a violent side-to-side oscillation — the 'death wobble' — usually triggered above 45–55 mph over rough pavement or an expansion joint. Mechanics are unanimous that a new steering damper (stabilizer) is a band-aid, not the fix; it quiets the truck for a month or two, then the wobble returns. The real causes are a worn track bar (the number-one culprit), ball joints, tie rods, and loose sector-shaft nuts. On your test drive, hit some rough pavement above 50 mph. If it shakes, budget for front-end work, not just a $60 stabilizer.
Sources: Independent mechanic and Ford-tech channel transcripts (Super Duty)
$150–$400
Track bar / stabilizer link
$1,500–$3,000
Full front-end rebuild