Give this guy a break. We all were not born with automotive knowledge. He is at least trying to learn and has an interest in maintaining his car.

To the original poster I would suggest you have your differential serviced by a shop. Its messy and difficult to get to under the car. It is good that your looking for knowledge on your car. Knowledge about your car will keep dishonest mechanics and repair shops out of your bank account.

I do a lot of DIY jobs myself and there are some things that I avoid. The messy jobs that require me to have a lift I try to avoid. I have my differential fluid changed by the quick lube place when I have the oil done. I do however bring my own oil and own oil filters and own gear oil. I don't trust my car to their cheapie filter and oils. Go to a place that will allow you to watch the process and by all means watch them work on your car. You might learn something in the process and keep them honest as well. Be cautious when going to quicklube places and find an honest one. My brother drives an older 300D and went to one place and had a certain job done on his car for a certain price (I forget what it was now). I roll up in my shiny Black S class to the same shop(an 82 300SD essentially the same car) and the price is suddenly considerably higher because they think I have deep pockets.

One of the most important aspects of DIY work is knowing what you can handle and what you can't. Nothing is worse than having to give up and have your car towed to a repair shop half assembled and have the mechanic sort out your failed DIY project. Or even worse is finding out you did something wrong when there is a catostrophic failure due to a simple ommission or mistake.