I'm having a similar issue.
It was there when I bought the car, just a slight vibration at highway speeds that comes from nowhere in particular. I feel it a little through the steering wheel, and a little through my butt. I replaced front suspension and axles because the boots and struts were bad, new brakes wheels and tires (not for this issue), did an alignment, and still feel this slight vibration. I need to check the rear driveshaft and axles for play.. but there's no wheel bearing hum so I don't think it's an outer bearing. I'm going to do fluids and inspection in the next couple weeks so we'll see if that narrows it down.
side note, had the wife in the car over the weekend and the vibration seemed less.. I usually drive by myself.
Did the OP find / fix the problem?
Last edited by bikesandcars; 01-29-2013 at 08:29 AM.