2006 Chevy 2500 diesel, at 45mph it feels like the truck has a flat spot on the tire or something like that. It is a prominent bump in the ride. It will do it even if I place it in N. The tires are new and it was doing it before I changed tires. The dealer said that it was a bad design in the gearing ratio. Baloney, it did not do it new. I was just wondering if you had heard this complaint before.

Everything that is rotating is suspect here: brake drums/rotors, drive shaft and both drive axles plus the rear differential.
If it's 4WD, you can add the front drive shaft and both front axles plus transfer case components and the front differential.
As you can see, there are many things that need ruled out during diagnosis.
You seem to have quite a trek ahead of you if that vibration is that noticeable.