The rear alignment should not be off that much. Not enough to make a difference anyway. The tires might be the cause if they are getting worn down or need rotated.

But my bet would be on the bearings. One way to tell if jack the car up and spin each wheel by hand. A bad bearing will make a noticable noise, even when spinning by hand. Or, if there is any play in the wheel, that might also be a good sign of a bad bearing.