The grub screw inside the gear box needs to be tightened. You'll need to remove the transmission to do this :(
Thoroughly clean and dry the grub screw and receptacle threads, and use red Loctite otherwise it will loosen up again sooner than you want.
It's a good time to do a full spline lube, check/replace the clutch if necessary, and replace the rear main seal and O-ring if leaking.