An out of balance tire OR a tire with a seperated tread is probably the problem. If your wheel wabbles left to right then it probably tread separation. If the car vabrates to the point where you can feel it in you calf muscles than it is probably balance.

Rotating the tires front to back will help you determin which tire it is. Check also that there is no 'Fix-A-Flat' or similar sealant in the tires. If you still have a vibration go to a proper tire shop-not Wal-Mart- and have them balance the two tires on the front. If this doesn't clear up the problem then get one of the tires of the rear balanced. If this doen't work then the chances are that the out of round or out of balnce tire is the one left. If balancing all the tires doesn't work you probably have tread separation. A good tire shop should be able to find the problem very quickly. Don't let the wabble go for too long. I lost 3 high dollar tires to tread separation (one at over 90 phm) all within about 2 months. The tread hit the trunk floor so hard that it dented the battery box. It also tightened my ass like nothing I have ever experienced before.

Good luck.