Here are my answers:

1. Do I need to change the washer on both the fill plug and drain plug or just the fill plug?

The differential's fill and drain plugs do not have any washer. You only need a 14 mm allen head to unscrew both plugs. Be sure to remove the fill plug first!

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2. Is this washer available at Autozone, Kragen, Pep Boys or is it only available at the dealer?

No washer is needed.

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3. Also, for the power steering fluid, can I use any popular brand power steering fluid or do I have to use MB steering fluid or Mobil ATF D as specified in my manual?

Just use any popular brand PS fluid. I prefer Valvoline SynPower PS Fluid. DO NOT use ATF as modern ATF have additives that may kill your power steering pump's seals.

Hope this helps.

Eric