I'd throw the bike on a stand and check rear wheel alignment first. Does the chain look like it's running at an angle from the rear sprocket to the front? Put a straight edge ruler or a level (from a stationary point, like a few links back on the sprocket) beside it if you can't be sure. Next I'd go after wheel balancing. Throw it on a stand and make sure that's right. If neither, then go after steering bearings.