Read you bought it missing the nut , I would go with GRT's thought , when the nut was loose , it wallowed out the hub and most likely damaged the splines on the axle ----if you find that is the case there a couple of shade tree mechanic ways to extend it's life , one is to clean the splines of both the hub and axle and apply some CRC Minute Mend or JBWeld on the splines and tight it up and let it set or weld the hub to the axle ----if you need a new axle and hub anyways