The rear tires could easily be the issue as stated above...degenerating plies, or even polyester cords can cause this. Any serious previous alignment or balancing problems could have left the rear tires cupped or otherwise unevenly worn which is what you could be hearing and feeling.

Otherwise the hanger bearing is a good place to start, and the other thing that can happen over time is that the driveshaft splines themselves get worn, and have excessive play which generates the sort of slight vibration you might be feeling hearing. The spline/shaft alignment should only be an issue if the driveshaft has been taken apart for some reason.