Ford has some 'better ideas' like no grease trunions (U-joints) and other stuff, so does Chevy and Fiat-Chrysler, not just Ford. It's all about making it as cheaply as possible and selling it for the most profit, the American way.....
I still believe it's the tires but I'd be replacing the trunions anyway with Super U-Joints. They have a grease zerk.
I had to replace the upper and lower ball joints on my F350 (solid axle Dana 60) in the first 30,000 miles because they aren't greasable from the factory and they got wonky... Not a big deal, just time consuming. All have zerks now. Still a PITA to grease the uppers as there isn't sufficient room to leave the zerk in the upper without the axle trunion wiping it off, so it's a pull the plug, screw in a zerk, grease and remove and replace the plug.
Ford also puts a viscous dampener on the rear propeller shaft that sometimes gets wonky and that will set up a vibration as well. Problem is, thats a driveline specialist replacement as it's pressed on the propeller shaft. I'm on my 3rd carrier bearing in 90K miles. At least Ford sees fit to apply super slide blue to the splines.
It's nice owning a machine and fabrication shop. I replaced every suspension spring pivot with roller bearings and cross drilled securement pins with grease fittings and flipped the front springs so the swing shackle is in the rear instead of the front.... Ford did that on subsequent Super Duty trucks, must have looked at mine...lol Still rides like a lumber wagon, it's a one ton so I don't expect a Caddilac ride but at least you can drive down the road without things flying around in the cab.
One thing I forgot to mention is that you have to 'time' the driveline if you remove any component for replacement/rebuilding. The easiest way is to pre-mark each section with a paint marker so when you reinstall it, you can line everything up. If you don't, it will vibrate at speed. If you had it apart and never marked it, it's easy to tell if it's 'timed' by looking at the position of each trunion. Each trunion starting from the back differential has to be at a 90 degree relationship to the next one (looking at the outer knuckles). They can't run parallel, they will vibrate. opposing knuckles cancel any imbalance.
Finally, make sure the weight tab is on each section. There will be a spot welded on weight tab on each tube from the factory and when working on a drive shaft, it's easy to knock it off. If it's gone, the driveline will vibrate. If it is missing, you'll have to have that section harmonically balanced at a driveline shop.
Keep in midn that if you have say, a 3.55-1 final drive, that driveline is turning 3.55 revolutions to turn the tires 1 revolution. That baby (driveline) is whipping around under there at 65 per.
I just muddied the water a bit.....