Damaged Wheel

some drive shafts have an "intermediate" bearing about half way down the length of the drive shaft, then that is bolted to the frame.

 

you may have all this done, but at anytime, were ALL the wheels checked for either out of round (true using a dial indicator?) or wobble as in even slightly bent?

 

applies to cars mostly, but can lead to a diagnosis for you as well

Diagnosing Vibration Issues - Tire Review Magazine

 

you overlooked the muffler bearings and you're low on 5th wheel fluid

 

The Samson tires would be the first thing I would change. Get some good quality rubber and it will make a difference

 

Make up your mind, at the beginning, you said all 10 wheels have been replaced and balanced, now your saying only the front 2 have been balanced, so which is it ?
Anyways..... if you have done all the above, what’s left is you might have and out of round tire. I had one and it would vibrate between 58 to 65 mph. The sad part was I couldn’t believe it was a USA brand tire. So that’s one option you can look into. The other would be the angle of the driveline. That would cause a vibration as well. You said you check the air bags height, beware that they are several heights specifications available, depends on the suspension you have.