Lateral run out is most likely at the drive/swing arm junction. Those bearings are known to wear and beat dimples in the races. You can verify by having some one move the wheel while you (with the boot pulled back) watch/feel the attach point for the drive. Easy fix - reset the preload or replace bearings and reset the preload. DIY for about $60.00. If your source of play IS the rear drive you are already toast as they are tight right up to massive failure.