Have you tried just loosening and adjusting the axle assembly? There is usually some slop in the bolt holes to allow for adjustment.

If that doesn't work, you can buy shims to go between the hubs and the axle to adjust the alignment angles. Probably a lot cheaper and easier than replacing the whole axle assembly.

Did you hit a curb or something to knock it out that far? If so, listen for a failing rear wheel bearing, it doesn't take much to damage one.