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I have a vibration that starts at about 25 mph and continues to at least 80 mph. The vibration itself isn't too bad - buzzing in the floorboards, seats, etc - but it's accompanied by a very annoying low frequency growl. It is only present under load, when I lift off the accelerator the vibration/noise go away entirely.
I rotated the tires, I also changed out the rear diff fluid and saw nothing out of the ordinary there. This pretty much leaves u-joints unless someone has other thoughts. The rear u-joints were last changed 53K miles back.
'02 Excursion Limited 4WD
7.3L Powerstroke
292,000 miles