What were your symptoms with this HB? I am thinking my HB is causing my problems but my problems are INTERMITTENT and I am not sure that a bad balancer or bad mount would cause INTERMITTENT shaking....my HB isn't in the greatest shape and my mounts seem to be slightly cracking as well....
I have the exact same issue with the shimmy at idle felt randomly intermittently...it is felt through the seat/floorpan and is more of a shake than a vibration.
Have you fixed this problem?
Do your rpms change when this happens? My rpms stay the same but I don't know if it could still be...