Vibration under acceleration

[cat6nz]

Hi all i would appreciate any feedback regarding my problem, i have a 2013 glider 389 pete powered by cat 6nz with 560k miles. Im second owner of this truck and since i got it i have a vibration kind of shudder feeling under heave acceleration, specifically i feel the most when im fully loaded uphill, boost its within specs (28-30psi) when i let off the accelerate vibration goes away and becomes smooth, my rears drive line truck height and all that its inspected and all checks out ok did an injector cut out test and they all sound the same. When im stopped clutch engaged and rev the engine to 1300-1500 rpm have some light vibration compare to the one loaded. I read almost every post related to my problem, and in one of them rawze with unilivers were thinking that a low crank engle might cause this problem, im not a mechanic and dont understand much but im doing a lot of research trying to get this problem solved, their description fits my problem. my engine is tuned up 6 years ago by Pittsburgh power.i got a 550hp tune on it, they were both saying to back off map timming by 2-3 degree and the problem will be solved, why 2-3 degree?? this monday i have an appointment with a diesel injector shop which got a good reputation. Before taking my truck there i want to know as much as possible so that way i know what to talk to those guys up there and also make sure that they will not lean my problem to drive tires and get away with it ;) any experience any ideas is highly appreciated...