If you are going to go to 3:73 or 3:42 gears. I would buy a new Posi like a Eaton or Detroit True Trac. There is also a rear end cover that you should add to the rear end that has baring cap supports. Don't for get the reluctor for the ABS system. If you go all the way you will spend $1200-$1300.. I would look to a shop that does drive train work that does drive shafts and transmissions..
Good luck! You don't want a rear-end that whines...
my suggestion is always the same for buying new parts
look on craigslist.com near you or maybe a few state away for someone that has already spent BIG BUCKs on NEW stuff that never finished there project or who knows u might find a built posi that u can give the guy your rear end as a core charge for less money you know just a though and my .02