I agree with the others that changing your plugs out is definitely something just about anyone could do, although you have to have a little bit of sense so you don't cross thread and/or snap off a plug in there. With that being said, however, it's definitely a good way to save yourself as much as $200.
As for the gear oil, that's also something I do myself but it's a little bit on the messy side. It costs me about $80-$90 to purchase the appropriate quantity of Mobil 1 and the necessary crush washers, and after that (assuming you have a drain pan and the hand pump to refill) should take around an hour or so to complete all three. Again, you could save another $150-$200 doing this yourself.