R7-play in rotor gearbox input sheave

There is enough play between the rotor gearbox input shaft and the torque sensing hub that I'm getting a definite rumble. There is a slight amount of rotation possible between the shaft a hub and it will also move in and out. The nut is tight. I had everything off the shaft about eight hundred acres ago (to replace the oil seal) and it all seemed alright. Is the shaft worn? The hub? Everything? Can one shim behind the washer to take the play out? I have thirty plus years of experience with L's but this is only my second fall harvest with this machine.