I had the same thing happen to my car. first i took it in to get a balance and they did that and told me i had 3 bent rims. so i got 3 new rims. it was better but my car still vibrated. finnally i took it to the shop and they jacked the front of my car up ran it up to 80 and they could see the tires wobbling in and out. basicaly the tread had separeted on my front tires. fortunetly they were under warranty so i got then replaced for free. but if balancing your tires doesn't work and none of your rims are bent you might get someone to check your car out like they did mine. tire shops will say that they can test your tires on their little spinner tester thing but for my tires that wasn't fast enough. i had to go over 60 and i think their machines only go like 40-50. anyways hope this helps. good luck
the_bg