I've found that pretty much anything rubber in the suspension, steering, or driveline can cause shimmy. So pretty much everything you've mentioned could be your culprit.Jack up your front end and without taking the tire off, or even loosening the bolts, tug on the tire as much as you can. All over. Try to steer both wheels back and forth quickly. Technically, you should not feel ANY slop, bangs, noises(except the steering box spinning), anything. Do the same with the rear. The feeling in your seat means rear half of the car, if you feel it in the steering, then the front half. Both...Both. Whatever it is, have patience, but get to it. a shimmy in one place can easily cause other components to fail prematurely. The domino effect. Good Luck,