Nissan Altima

I have a 2001 Nissan Altima with 86,000 miles on it. I noticed that my car was pulling a lot to one side, so I took it in for a wheel alignment. The shop realignmed my car and had me get two new front tires. After the alignment, my car drove straight, but I noticed a lot of vibration in the steering wheel and seat. I took it in to a different place to get all four tires balanced, and it helped, but the problem is still there after I go above 60 mph. Is there vibration because of uneven tire wear, since I have two new tires and two old ones, or is there another issue here? Thank you.
June 3, 2007.

Why didn't you take it back to the first shop?

It sounds like a bent rim, tire balance problem. If you feel it in the steering wheel, then it is in the front. The new tires need to be checked for a wobble, rim problem or balance issue. The first shop I would expect to do this at no charge, unless somethinng else is unlikely uncovered.