you checked your swivel preload, your panhard rod bushes , your wheel bearings - it may be the damper but dont be tempted to put on one of those POS RtC dampers they just mask the underlying issue for a period of time.
sounds like the dreaded death shimmy- which can be any number of things get the thing off the deck and get under it with a pry bar - then do your wheel bearings and swivel preload.
