2006 2500 4x4 5.9 290000m all stock engine

this is a disabled veterans truck i try to help him as much as i can but hes stubborn has had an exhaust leak on the # 6 cylinder for approximately 3 month wouldn't fix it or bring it to me to fix .past issues have been 2 sets of injectors replaced , tipm replaced then third set of injectors replace at dealer ,trans and tq converter rebuilt /replaced and #5 injector replaced .
now the truck has an engine vibration witch occurs when going from on the fuel to off throttle at all times under all conditions .

when he lets off the throttle the truck shakes and he can feel it thru the steering wheel he has determined its not suspension related as it reproduces at a stop . I am an hour drive away and this individual insists on drive way diagnosis smh

with the exhaust leak im leaning towards an internal issue ie burnt valve , melted injector tip etc before i drive 120 mile round trip to look at this truck what other things should i check besides the dampener ? im kind of leaning lo a lack of fuel , dampener ,or internal issue but any other ideas would be appreciated