TiPwEiGhT, like EricFerret says.
It makes sense that this could be the problem, since most others have been eliminated. You describe increased vibration in cruise, which suggests that when the tail rotor is asked to flap there is some non-linear behaviour in the tail rotor bearing. In flight it would be under load while moving, so could judder. Vibration would be almost at the low frequency end of audible - haven't got Prouty to hand so don't know TR RPM...
From ground vehicle design experience i know that elastomeric bearings fail by splitting the rubber. It takes a long time to start splitting, but once started does not take long to break up (from my experience that is not immediately catastrophic, but the bearing does start to float).