Sleeping big guy... I'm on the other side of the world.
I put Traxxas 10K diff fluid in both front and rear diffs (this isn't ideal - you probably want thicker in the front, thinner in the rear - especially if you are gonna race - 10K was all I had at the time) just buy some from your LHS. Its not exactly easy to put in, remove rear end from chassis, strip apart to extract diff - heat diff screws up to break locktite and open up, same for front. To lock the center you need to remove front and rear, engine, 2 speed, one side of chassis and extract transmission assembly, heat up diff screws again and remove gears and fit lock block.
Locking the center diff will cause more wear and tear on your drivetrain... but its a lot more fun! [>:]