The problem with a vibrating issue is that it could be so many different things, you're just going to have to take everything everyone tells you and start eliminating the variables.
In the case of my moms CC with a POS 6.9 we purchased as a rebuilt engine, it feels like you're breaking warp speed or something when you get over 2300rpm I've wrapped it out to 3500rpm just to see if it would come apart and it doesnt. I think its a combination of an unbalanced rotating assembly and the apparent lack of a thrust bearing in the mains.
As mentioned above the dual mass flywheel will make it vibrate if its gone bad, but if it has gone bad there's a reason for it, which could be another vibrating issue, when injectors get worn they dont wear evenly, and they pop at different pressures. This poping at random pressures makes the motor unbalanced in its own sense, because of the cylinders having slightly different firing times. This problem even happens with some NEW injectors, thats why its important to have a set of injectors pressure matched.