It could be worn bushings, but I think I would opt for one more go at the wheel balance, on the Hunter 9700 computer balancer. I almost threw away a perfectly good set of tires to buy a new set, because I had balanced them several times, different places, still no good. Then I found one of only three places in town, with such a machine, and had them balance the front ones only. I did not get 3 miles down the road before I felt a totally new car. No more vibration in the steering wheel only the seat and rear. Had the rear done the next day, and has been perfect ever since.
By the way the difference in the front two wheels only were almost 3 ounces off, after just having them balanced a few days earlier. Thats how good this machine is, provided the operator knows what hes doing.
1989 420SEL 105575 miles
1990 Chevrolet E-Cab C-1500
1989 Olds Toranado Trofeo'