After sleeping on it....
All of the rear suspension/rear end bolts should be double checked. A loose rear end can cause the steering wheel center to move around. Maybe make it feel loose???
The late 90's Ram 2500/3500 diesels all cam with a panhard bar/track locator on .the front diff.. I don't know about the newer ones. Maybe something to think about installing on yours???
If it were mine, I would have put on a GOOD, heavy duty steering stabilizer too. I've seen bad ones cause all sorts of weird ****. Vibrations, wandering.... Don't know- one could help....
I like everyone's ideas too. The box, frame, rag joint... Best bet would be to take it to someone different for an inspection. Get some different eyes on it, and a second opinion. Some experienced old fart, that won't run away from rigs like this....
TL;DR: Buy a new truck.
Lol. j/k....