Do you have after market wheels? Sometimes they are not quite so true as OEM wheels are. Also, it may be you've hit something and bent a wheel slightly. This is particularly possible if you have very low profile tires that are not inflated to the recommended level. There isn't much shock absorbing potential for a 35 aspect ratio tire side wall even when it is inflated properly.
Just some ideas. Musikmann's suggestion about wheel balance is most likely your problem; at least that's the place to start.
Good luck.
Just some ideas. Musikmann's suggestion about wheel balance is most likely your problem; at least that's the place to start.
Good luck.