I've had issues with "square" tires causing vibrations at certain speeds. The Goodyears that came on my Ford, the Duratracs that I put on my Jeep, and some others. And I actually have a vibration in my new 2500HD at 70 mph with the stock tires right now that I need to sort out.
- Tires have heavy and light spots, as do wheels. Sometimes the fix is to have the tire remounted on the wheel in a different orientation so the heavy parts of each are not aligned with each other. Look at how much wheel weight is needed to balance your tires currently and if it seems excessive, ask a good tire shop to re-do the job. There used to be dots on the tires and wheels that indicate the heavy spot, but I don't know if that's done any more.
- I also recall feeling that floaty feeling on my first set of duratracs (which I installed after a leveling kit on my F150). Part of the issue was that when you level the truck you also adjust the dynamics of how it handles. Lifting the front end allows more air under the truck, less over the top, which changes the downforce caused by the wind. A sensitive person might feel that. Also, the duratrac is known to feel floaty if it's under inflated due to how the sidewall moves. It can also feel that way if fully inflated (80psi) due to it having 19/32" tread depth and the knobs wiggle a bit between the tire body and the road. As the tire wears in and the contact patch grows a little, it'll feel more planted. I settled on 55-65psi in the tires I as a comfortable pressure with good handling, but that was a 33x12.50x18 tire on a eco-boosted F150, not a heavy diesel.
Please keep us informed as you track down your vibration. If it's not tire related, I think we'll all benefit from learning what the cause is.
- do you know how RAM balances their driveshafts? If it's with exterior weights, you might have lost one of those.
-Any chance you have worn rod ends on your drag link or tie rod? those could allow the tires to move and vibrate at speed.