I woould remove the rear wheel hub and check the grease in the bearing. You can clean it and repack it in the hub, but if you remove the bearing to repack it (as you should) be sure to have a new grease seal on hand. I don't know of any way to save the old grease seal when removing the bearing.
DO NOT drop the hub on the floor as suggested in some book to remove the bearing and seal. It takes prying and hammering to get the old seal out, and may take some sanding of the new seal outside diameter to get the new seal to fit. Be sure to clean the new seal of any sanding or grinding dust.