Suspension problem?

It could be a number of things, anywhere from loose lug nuts, a bearing, out of round tire, or the rear end. Try to eliminate one thing at a time. First check the tire and rim. If the tires are older than 10 years old check for out of round. Check any free play movement of the rear axle. And check the rear to make sure it has lube. Most likely I feel it will be in the wheel or rim.