Depends on the crush washer, what shape it is when you remove it. Normally, you can get away with one or two services, before that washer is too distorted to seal correctly. But like they say, your mileage may vary. Some people can reuse it - no issues, others will have it leak almost immediately. Shop manual recommends replacement of washer, given the time interval between services.
At my dealership - they usually drop in a couple washers for free - when I pickup a case of filters, oil, etc. Otherwise, it be $1.50 each.
As mentioned above - make sure you can remove the fill plug on the differential before you attempt to drain it. There are times where that plug will be so tight, that only an impact wrench will spin off the fill plug. Give it a good yourself - if it doesn't come loose - take it to a shop and have them loosen it for you. Many shops will do this for no charge, if you ask.