the rear axle is a one piece unit, I would say you have stripped the splines from one side of the axle, or you have a stripped out hub. take the hub off of the side that is no longer pulling(remove the cotter pin and one large nut) the hub will slide out off of the axle. look at the axle, and the inside of the hub. either the axle will be stripped, the hub, or both. very easy to fix. parts can be had a lot cheaper on ebay.