Vibratioin at 30psi and around 1300 rpm

[Trucker66]

I got a 2014 T680 with a Cummins and had it "tuned up" last summer.
During the tune up I had the HP bumped up from 485 to 550 hp and the torque bumped up to 1850 foot pounds.

Right after that I started getting this vibration on every hill once my hp drops to approx 1300 and my boost is always at 30 psi. As soon as I let off the throttle it will stop.

I can't figure out where the vibration is coming from. It feels like it's a torque issue but I have no idea where it's coming from. Maybe clutch, drive shaft? rear end?
I just turned 400,000 miles and I had the transmission oil and rear end oil changed and there were shavings on both rear end plugs..
On the valve cover it's rated 485 HP and 1850 ft # but it was set at 1750 from the factory.

The engine was rebuilt at 330,000 miles due to bad rings and liners and thats how I found this site. After the tune up its been running like a top, not a single check engine light. Just need to figure out this vibration issue.