At 55mph my truck feels like I'm driving on a rocky road

[XBRMann]

I have a 2003 F350 that has been working like a champ until recently. When I am driving down the highway and I hit about 55mph my truck starts to bump around as if I was driving over rocks on the highway. If I put it into tow/haul mode the issue goes away until I hit about 65mph and the same thing happens. I don't even know where to start on this one. Has anyone seen anything like this?

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