Better off not to shim it up to stop the rocking or bad things could happen to the circlip & groove. Never done it but machining & sleeving sounds the cure, then you could shim the side clearance later without side loading said circlip. Comon problem on all my old trail Yamahas too. Give me my "68 DT1 with it's sprocket mounted on it's own bearing/stub axle thingy, fail safe that way plus you could change the wheel without loosing sprocket & chain etc. Buggered if i know why they changed it as wasn't heavy or anything.