1994 Mercury Tracer

1994 Mercury Tracer with 145000 miles. Vibration and then shaking of the steering wheel at about 35-45 miles per hour, then stops once I am up to speed on the highway. Had the tires balanced and it seemed to help, but only for a couple of days. Is it possible that the weights just fell off and I need to have the tires balanced again? Or am I in for a major repair bill here? The mechanic did note that both rims were slightly bent. I did hit a major bump in some road construction that probably caused that.