bugdevhelo,
The wobble is probably not the shaft, but the actual tail cone,where the natural frequency of the structure is tickled by the rotor rpm at a certian speed on runup. Just walk up to the aircraft while parked (and shutdown, of course!), and shake it by the tail to see if it wiggles that way at that frequency. Most likely, you are seeing what many helos have at odd rotor frequencies. I recall that in Hueys you can hardly fill out the paperwork while at idel rotor, due to the vertical hop in the cockpit, due to the natural fuselage mode.