Internal balanced..what damper do I need

So my new 408 has a scat internal rotating Assembly, but I have a nasty shake. I'm still using a stock damper, is this wrong?
No matter what I do I can't find what's causing it. Only at idle, or slowing down while driving(coming to a stop). Motor mounts look good I also have a torque strap. My air fuel ratio is good. New wires new ignition.