I'm kinda in the same boat as you. This is my 2nd 2500, both with the 6.4 motor. Prior to that I had a couple of 1500's.
My 15' 2500 was bought used and I put 295/70/18 Ridge Grapplers on it. They gave me a rough ride and when not towing I ran them with 50psi Front and 45psi rear to give me a smoother ride. It really didn't wander, but on a couple of occasions I had excessive vibration either coming from the front or back of the truck and I would take it in and get the tires balanced and it would go away. When I sold the camper a friend of the family wanted the truck as a package deal so I sold the truck with 31k on the odometer.
I replaced it with a 17' with 11k on it. It rode fairly smooth and I got 17.5 mph across country when I drove it home. That Winter it was slipping and sliding around on those Firestones so I decided to upgrade the tires and had 295/70/18 Ridge Grapplers installed. Well, they vibrated like crazy and the wheels were lined with wheel weights. Long story short, after Discount Tire could not get the vibration to stop at highway speeds they let me change the tires to Cooper At3s.
Right away the ride was much smoother. Of course the Coopers only have a 2 ply sidewall, but they still have the 4000# load rating. It wasn't much longer that the vibrating at highway speeds started happening again. I went in for a rebalance and the ride was smoother. They suggested that maybe the left wheel had some slop in it so I had it checked and aligned at a local Dodge dealership who found nothing.
Once again a few months later there is some vibration at speeds greater than 61mph. With 20k on the Odometer I am not sure what to do. I am thinking of a Thuren Suspension upgrade with the softer springs, track bar, and shocks. Before I do that I am considering spending an hour of shop labor at my other Ram dealer where I get my oil changes. When I was having trouble with the vibration from the Ridge Grapplers they said they could swap out the wheels with a truck on the lot and see if the vibration goes away.