Installing a rear end is very hard if you have never done one before. I think you should take it somewhere and have it done. If you don't adjust the backlashing and align the teeth just right, it will start to whine and chew up the teeth.
I had my 3.73 installed professionally. Haven't had a problem since. 300-400 just for labor is pretty expensive, but it's litterally an all day install. I spent about 400-450 (with parts) for mine. Look around at other performance shops. I would also go with a performance shop as opposed to a "normal" shop.