Using different model tires front and rear won't cause vibration you feel through the steering wheel. Out of balance tire(s) would, but since you already got them balanced, I don't think rebalancing them again will help. Are you feeling serious shaking vibrations, or are they road feedback type vibrations.
There is something called road force variation balancing; balancing the tire under load. Most Discount Tire stores have the Hunter 9700 which can balance using road force variation. It cost $80 for 4 tires when I got it done. This may or may not help.
http://www.sullivantire.com/xcessorize/hunter.aspx
Maybe you need an alignment? How does the tread wear look? I recently got a wheel alignment, but it was because of my tire wear, not vibrations.
Good Luck,
Larez2
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