Trans/Clutch.

A lot of problems can cause a vibration that might seem to be in the transmission when under power: drive shaft issues-needs balancin and carrier bearing; crankshaft dampener; flywheel needs turning; and even collapsing fuel lines. I would start with all the stuff that doesn't require the tyranny to be dropped. Drop the drive shafts -take all 3 to a drives hat shop. Install new carrier bearing and dampener, and replace fuel lines. Unless you know for sure any of these have been done in the last 500k-750k miles, and it doesn't fix the problem, it's good maintenance practice anyhow and should be done.

 

Make sure your truck is sitting level if not it could cause some of those problems...

 

I agree with Final Drive, ride height too high will cause grinding while accelerating, or slow rolling forward, too low causes grinding while coasting, or slow rolling in reverse. That’s been my finding anyways. Worth ruling out.

 

Just for hahas and giggles park the truck on level ground with wheels chocked and brakes released. go around measure the over height of the air bags from paddle to frame. 11 inches is peterbilt spec for air leaf and air track if that is what you have for suspension. Suspension height is adjusted thru the leveling valve. You would be surprised how a little bit of suspension height one way or the other will affect peterbilt drive line angles and create rubbles and shakes that you could spend a ton of money chasing