IF after balancing and rotating the tires and the problem persists then you should check for a bent axle or axle flange.
This can be done by jacking up the rear differential and removing the wheel on the passenger rear.
Chock the front tires and place jack stands under the rear differential before removing the tire.
Remove the brake drum if you have rear drum brakes.
Spin the drivers side tire holding a piece of chalk near the passenger side axle flange and check for consistent contact (360*) on the flange face.
While spinning the axle listen closely for grinding sounds; your axle bearing could be badly worn causing the wobble.