471 Detroit Diesel Shake problem

Sounds like bad motor mounts or something much more serious (bent rod, bad cam, etc). Don't read too much into vibration not appearing at low RPM, where it's not noticeable or at high RPM where the speed smooths it out. One indicator of what's going on will be when you first noticed it (was it right after winter which might indicate the mounts, or after pushing high RPMs or striking bottom which indicate internal damage).

Maybe someone here can help. Anyone know of a good DD mechanic in the Philadelphia / Camden N.J. area?
In the mean time bring it up to where you've got a good vibration and check your motor mounts to see if one or two seem to be vibrating more. Even if it's that you'll need help changing or adjusting them. If it's internal you'll need that mechanic, if for nothing else then to give you peace of mind that it's normal.

Hard to imagine it is a motor mount given it does it in neutral at the dock.

You should have no trouble finding a DD mechanic there. Check Utsch's, the commercial docks and West Marine for recommendations.

Actually that what leads me to hope that's what it is. We know it's not the running gear. So if not the motor mounts it gets very expensive.

When in neutral there should be virtually no torsional resistance (other than weight of crankshaft and items that rotate when in neutral) to impart load into the mounts. It should be pretty limited to the dead weight of the engine. Thus my comment that it is hard to imagine how an engine mount could be the issue in this case.

You may very well be right. It's impossible to diagnose with a keyboard. I'm jut pointing him to possibilities that he can check himself. The mounts would be the easiest to to check. One other cheap possibility would be bad injectors causing the motor to run rough. After that he's heading to the mechanic.
Btw, always try to observe work that's done. Consider it going to school along with keeping everyone honest.