[jmitro]

it's not rear wheel bearings or toe out. non-hubcentric spacers is a good thought.
sounds like a tough one. What about the rims themselves being out of balance?

is it RPM dependent or strictly speed dependent? Front or rear of the car?

EDIT - just read the symptoms in the steering. Maybe it's a bad steering rack or steering linkage?

these may be long shots but my only other thoughts are a bent spindle on the front wheel carrier, possibly bent axles? or possibly bad motor/gearbox mounts. On my BMW M3 I had a really bad driveline vibration that finally turned out to be a shot differential mount