I would check all suspension components under the front or have someone do it. Some folks can get close guesses by examining tire wear. Worn ball joints can also cause shimmy when traveling and braking. Worn shocks can cause these type problems but usually not severe. Just get under it and start twisting and pulling. You can check ball joints slop by pulling on either front tire from the sides (front to back) any questionable movement is suspect to replacement. And remember, driveability and comfort is only as good as the equipment your running. Worn tires and suspension components cancel out balence, alignment, and such.