sh*t... I had a viabration coming through my steering at 100 - 110, it annoyed the sh*t out of me so I know what its like, took my dealer / tyre shop 4 , YES 4 attempts to fix this, it was a balancing issue. The only other issue ive heard are the following. A) Tyres can get a flat spot if your parked for a long period of time, this causes a viabration through the wheel till they heat up. B) The tailshaft is missing a weight, which needs to be rebalanced, but if this was the case stuffing around with wheel balancing wouldnt fix it, so it seems like its a wheel balance issue. C) Maybe you should get them to check all wheels again to see if they are bent, usually if you have one wheel that's bent the other wheel on the same side will be bent to as its probarly hit the same pothole/whatever caused the other bend.. If its none of these then perhaps theres an issue with the diff / rear suspension that needs to be rectified.