It's not hard. When re-tightening the nut, you'll want to check for in-out play in the pinion. Just make sure there is no in-out play in the pinion but you should still be able to turn it easily. If you have any questions, just let me know. I do some axle work here and there.
Terry Offroading: The hobby of turning perfectly good vehicles into scrap metal.
Just like Terry said, all I have to add is make sure the sealing surfaces are both really clean before you put the new seal in otherwise you'll end up doing it twice. Then again that may be common sense for everyone who isn't me.
