I've been lamenting my 90mph vibration and finally got it fixed, thanks to Rene Huiskamp who has posted about a similar vibration in his XC.

I had the wheels/tires balanced ON-vehicle and although there is still a little bit of vibration, it is much, much, much better and completely tolerable.

Some observations:

1) Tire balancing does not require a great deal of precision. The tools that were used for my on-vehicle balancing identify if there is vibration, but does not say the exact position or magnitude of the imbalance -- the mechanic is left to guess where to put the weight and how much to use. Nevertheless, the result was the best balance I ever got since my car was new.

2) Even though I felt most of the vibration at the steering wheel, it turns out that most of my problem originated from the REAR wheels! As they spun up each wheel, one at a time, you could feel the steering wheel vibrating when there was an imbalance. The vibration was greatest when the rear wheels were being spun. There was virtually no balance correction required for the front wheels (all tires had been previously balanced off-vehicle).

3) I had to call a lot of people to find this place. No garage likes to do it because it is a trial-and-error process that is terribly time consuming. One tire took an hour of tinkering. The others took 10 minutes each. If you're looking, good luck finding a garage that does this.

4) Don't forget to index the wheels to the rotors (paint-pen). You'll have to redo the balance if you ever change the position of the wheel relative to the rotor/hub.