ok lets start at the beginning it could be a stone /chipping jammed between the brake caliper/pads and disc, check that pronto or it could wear a groove in ya disc if it is that, bearings catch hold of the wheel top and bottom and see if it has any movement, if so it would be fair to say they are shot , replacing rears should not be too much more complicated than fronts, not sure of the set up of your bike but there maybe a bearing in the sprocket carrier as well, i reckon it would be worth replacing that at the same time