remove and refurbish... basically needs to have new bearings and seals put in... Including parts this job normally comes to around 3-400 at a good indy.
You'd be better off getting either your one reconditioned or buying a reconditioned unit if you wish to keep the car.
Buying diffs from a breakers is always a big gamble, as if the car / model is old there is always the chance that the diff is worn, and to be honest with most of these places you just have to go by their word, as you can't inspect the car.
If you are then unfortunate to get another one that is not up to scratch, you land up spending a lot more in labour and parts to get is sorted properly.
If you want to keep the car, just get the job done properly and in the long run save yourself the hassstle and money.
Here's a post detailing what might be involved in a DIY swap. Different car I know but gives an indication of the work that might be involved! PeachPartsWiki: Replacing the Differential