I might have missed it above, but are you sure your tail rotor shaft is not bent?  I had the same problem, with the V-stab vibrating on my 450 and it was bad enough that I could even see it in the landing gear as well.  I  couldn't actually see the bent tail shaft but when I replaced it, the vibration went away.

Also, A couple of flights after this shaft swap out, my battery came loose and wound up in the dirt.  The good shaft was now bent very bad and so I swapped the shafts back to the one I was having the vibration with and the vibration came back.  I used a probe to see if I could see the shaft wobble but even with that, I couldn't see it the bent shaft. 

My suggestion would ne try another shaft.  They are cheap and if that isnt the issue you will have a spare in the event you need one one day.