Good idea, not dumb, Nate. Thx I'll look into that. Seems a slim possibility, given the force with which it can vibrate the whole car, but worth a look.
Jason, its not just at idle but really any multiple of 1500rpm. Its worst at 1500rpm but it has some harmonic vibration on rpm multiples above that.
Reason I don't think its the tranny is it does it even when not in gear, but just sitting in neutral too.