I'm saying the wheel itself, the metal part is either out of ballance, or is bent a little bit.

 

It could be the flux compasitor, you should check that. My 04 six speed had the same problem but only when I had the truck camper on it, or really pushing it , the re-shimmed the rearend and it went right away.

 

Looks like placing a wedge between the springs and the rear axle change the start of the vibration point about 5 mph. Yes, it was and is a bandaid but at this point it does work and I did tow my trailer today and make sure everything works ok. I was also thinking I could put a little bigger tires on it but, we will see. At this point I either get rid of the truck because of the vibration or keep it and put up with the minor vibration that now starts at 70 mph. I think I will keep it. Love the truck over all.