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...