I don't believe the manual states a spec on the cush drive free-play. But, with the rear wheel lifted and the bike in gear, if you can rock your wheel back and for more than 1" or so it might be time to replace the cush drive.
I just took a piece of vinyl baseboard, cut shims to take the the slop and slipped them in front of the rubber shim.
This 95% of the slop out of the drivetrain and I have had no vibration issues. It cost me about $4 at Lowes.
It is a little fussy getting the shims in place.
I have pix somewhere.
