have you removed the final drive from the swing arm yet? Its a pretty solid unit. More likely the cause is the splines and coupler (42034) and the splines on the input shaft/pinion gear. if thats the case, scour the internet for a replacement (used of course) rear final drive assembly that has good splines, and a new coupler.
its a known problem, someone on the assembly line was sleeping for a few years and didnt get any grease (or minimal) on them during assembly. We recommend inspection of the coupler and splines ANY time the rear wheel is removed.