Chasing down vibrations, 98 ws 12.7 60 series

If it were mine I would switch out the cab and rear motor mounts for rubber and see if it helps.

 

Thinking it might be your out put shaft bearing. And check you air bag height when loaded. Might be out of adjustment. Causing your u joints to be at funny angle.

 

Did you through all that money at it because of the vibration? Always had it or an idea of when it changed? The parts replacement program could help someone put a finger on it. I thank everyone for their posts some good info and I will add one. Saw a truck with a 15.5" clutch that the shoulders on mounting bolts bottomed on the flywheel, after it was resurfaced, before the pressure plate was tight on the flywheel.
No gap at top of flywheel and bellhousing?

 

Put a new damper on it . It could be original for all you know.
There not terribly expensive and if the fluid inside has gone hard that would be your issue

 

I'd agree with the damper except for the fuel mileage jump. Something funky going on to cause that.

 

For some reason i am leaning toward the mounts

 

Sorry, the way I read it, there was no tie between new parts and the mileage increase.

If the damper isn't the issue, and the dyno is okay, stop the tranny with the clutch brake. If the vibration is still there, I'd bet something went wrong installing the clutch.