One problem related to rear suspension shock absorber has been reported for the 2006 Ford F-250. The most recently reported issues are listed below.
2006 F-250 super duty crew cab short bed with 5. 4l front end has a bad vibration/rattle when speed exceeds 60 mph. I drive 15 minutes to work each day on the highway and during my travels I bet whether or not the rattle and vibration will show up while en route. I notice the rattle even at low speeds in town especially on patched roads or pot hole type dips. At high speeds the vibration may last for 5 miles or more or it may clear up in a few hundred feet depending on how the next bump in the road strikes the front tires. I share the sentiment of other that have reported this problem that is an issue if you cannot maintain the minimum highway speed safely in your vehicle. Ford needs to investigate the issue. Whether its bad harmonics with certain shock absorbers, to hard of tires, front hub assembly bearing issues,toe and camber adjustments getting messed up, etc. There has got to be a component or set of related items causing what we all are experiencing.