Jack up a wheel and rotate it as fast as you can with one hand, holding a coil of the suspension spring lightly in your other hand's finger-tips. Is it a leaf spring, well try holding a thin part, out towards the tip. Maybe it will work on leafs as well as coils. If the bearings is bad you will feel the vibration, even if you can't hear it. Be sure to try the other side, maybe even the fronts too, to get some comparisons. I know it will be a pain to cut open those old bushings, but I think several of us are eager to see how the parts are worn.