the oil in the rear dif is for life, you should not need to change it. Are you under waranty still? I would talk to the dealer if you are.... if not, crawl under the rear end and inspect the bushings in the control arms, a car with only 36K on it should have bushings in good condition still. no rips or tears in the bushings or cracks that go all the way through. They should also not look overly squished, smashed or be bulging on the sides. it could also be the rubber isolators that your rear springs sit on. there not as easy to check but if you can find a way to see them then they should be in good condition as well.
do you have any mod's to the rear end?