Could be that your wheels are out of balance or out of round. That is pretty common on dirt bikes, but since most people don't take them to high speeds of 60+ mph on hard pack roads, they just don't notice. It could also be that one side of the rear axle is further back than the other - take a tape measure and check distance to the center of the axle on each side from the swingarm pivot.
There are a lot of other things that could contrbute to your problem, such as bad swing arm bearings, wheel bearings, or steering head bearings. If the bike is newer and in good shape, you can rule some of these things out.