Thanks stockman, I think we need a really simple way to isolate components until we have found which one is causing the problem. You could start by removing the complete cutter disk, including blades, and see if that eliminates the vibration. If it does, you could then remove the blades and put the bare disk back on, and see if the vibration is caused by the disk. If it still doesn't vibrate with the disk on, I think you'd need some new blades and/or blade mounting hardware.
On the other hand, if it vibrates with the disk removed, we'd be looking for a problem in the spindle, spindle mounting, cutter belt tensioner, or the deck mounting. We can work out a sequence for testing those bits if the cutter disk and blades turn out not to be to blame.
