Vibration Fun

Oi vey all... for the past month or so, I've had vibration when cruising at highway speeds (50-65ish) ... the best way I can describe it is to say that it feels like one of those electric massage chairs. A very rapid tickeling that I feel through my seat back. I've had the wheels balanced, checked the alignment, its still there. I searched the archive, but there were so many possibilities presented that I dont really know where to start. Should I just take it to the dealer and say "Fix it" or is there something more specific I can look for?

Any input would be extremely helpful...
-Colin