Vibrations at speed tends to be a balance issue. Is the vibration felt through the steering wheel, or through the seat? If it is in the seat, your rear driveline u-joints could be going bad, or the driveline could be out of balanced. Have you had the wheels looked at?, you could have a bent rim. Did they rebalance the tires?, or rotate them to see if the vibrations moved?, you could have a bad tire.

don't think the control arm bushings would cause vibrations, they would tend to cause a clunking noise when taking off and comming to a stop, poor tire wear, a bit of wandering on the highway, and as you know alignment issue.