it is inherent in a alot of the 13 F150's. I had it around 50-55mph. it happen more in winter time than in summer. I think it is either cold tires or ungreased/out of balanced driveshaft. on my truck it always was worse first thing in morning in cold weather, seemed once truck warmed up I was less noticeable but it never went away on my truck. But I no longer own that problem anymore..