Knowing what engine you have helps.
You'll want any one of 4.10:1, 4.30:1 (2WD only) or 4.56:1 with 30" tires on a 3.0L (go toward a lower number for highway driving (MPG), higher number if you want more torque for towing/acceleration, etc).
Cost usually runs around $500-700 per axle for all parts & labor at most shops.