Re: rear differential
Welcome to the garage.
The solution depends on where its leaking. Could be leaking from the differential cover. In that case the cover has to be removed, cleaned and new RTV applied and new fluid added.
Could be leaking from the pinion seal at the front of the differential. New seal will be necessary.
Or it will leak from one (or both) of the axle tubes. In that case the axle seals must be replaced.