Toyota Repair: steering wheel, balance problem, steering wheel

Question
I have a 98 4-runner and the tires were changed approximately 2k miles ago. I have noticed the steering wheel has started shaking. The shaking is directly proportional to the speed. I have had the tires balanced multiple times to no avail. What else am I looking at as the possible culprit?

Answer
It's not a tire balance problem, either one of two things is happening, there is either a bent rim or one of the tires is defective and may nhave tread seperation or a flat spot, have you tried rotating the tires from the fron to rear to see if it helped the condition, is the steering wheel shaking present at lower speeds also or only at higher speeds?