Rearend repair is a complex and tedious task best left to a qualified master of this craft! Truth, most mechanics today do not know how to repair differentials. They send these jobs out to a differential rebuilding shop. I know it sounds unbelievable, but for this job you require a specialist. Many factors go into securing a quiet and long lasting service job on a rear differential!
I will elaborate further, you will require premium parts Timken bearings National seals for an American made car or Koyo bearings and NAK seals for a Japanese make vehicle. Ring and pinion sets factory oem or original from the factory work best. Second best are the aftermarket such as Motive, Richmond, and US gear. Gear oil choice is a primary concern for gear life I recommend 85-140wtgear oil. Synthetic gear oil simply has insufficient cushion for the impact between the ring and pinion. Great for an engine but rearends need a thick like molasses gear oil! This will give you long life on your gear set in your rearend of your car!
Four wheel drive trucks racing applications normally do not come with a warranty! Its best if you go to a differential specialist to have your differential custom built for these applications. Always go to someone who does this work as a specialist. Most repair shops do this work one here and one there. You want someone who does this daily. Break in period is critical! no excessive speeds over 75mph for the first 500 miles and make sure your differential repair shop runs the differential in with no load! Meaning just let the rear wheels turn above the ground slightly above idle before the vehicle is driven. Thanks for reading!