Driveline vibration?

The truck had a set of Dry rot Michlins on it. I just put a new set of Goodyear Wranglers on it. It had the vibration with the old tires just as it does now. Idon't think it is a tire or wheel but tomorrow I'm going to rotate the rear shaft 180 and see if that helps, it may have been put back in wrong the last time the u-joints were replaced. Pinion angle looks good as I thought of that already. Truck is 100% original, not lifted. Even has the stock size tires on it. I runs so smooth until you hit 35-45 then you feel it in the seat and all through the truck, but not through the steering wheel. Hell I'll figure it out sooner or later. Thanks for all the tips Guys!