Depends whats wrong with your current diff if anything. I doubt the internal diff gears are worn, its more likley that the crownwheel/pinion have some slack due to wear if you are still sure it is still has too much wear. In that case its a full rebuild with either new parts and lots of time shiming testing and re shimming can be done but lots of people are not happy doing this and send it off to a specialist. Replacing the internal gears with a locker won't help if the slack is present in the crown wheel/pinion.
Well If you hear of any cheap even broken salisbury axles going that are disk braked axles (must be the bolt on caliper bracket type and not later welded type) I am looking for one failing that its £130 for parts to build my one up to spec.