rear differential gasket

The differential is that big pumpkin located in the center of your rear axle. It is an ingenious invention that transfers the power from the drive shaft out to the rear wheels. When you are going around a corner the rear wheels rotate at different speeds. If the differential were not there to allow this to occur then one of the tires would "hop"; llike driving a go-cart. Check out http://auto.howstuffworks.com/differential.htm for a better description.

The gasket leaking is not all that uncommon and it should be replaced. With that many miles you should replace the gear oil in the differential anyway, and changing the gasket would accomplish this as well. As for cost, it is hard to say but I doubt that it would cost more than $100