Some posi rear axles have clutches in them thats when you need the additives, you can get a bottle from any GM dealer, dump it in then after 20 miles find a large parking lot and make about 20 figure 8's... when you turn the clutches slip because of the difference in inside outside tire speed, when they slip they will be subject to the friction modifier you added to the lube and mixed up by driving the 20 miles, once the clutch plates are covered in the new oil additive any noises and chatter will go away.
Some ask why you just cant turn left for 10 loops then switch and turn right for 10 loops, GM recommends figure 8's so you dont over heat the clutches your trying to lube, switching directions gives time for the clutches to cool.
"well being that ive gone through 4 rears il help out"
I assume you went about 800k miles to go through 4 rears?
What ever you guys do after changing your axle lube and filling up after taking it for say a 100 mile spin soon as you stop open the fill plug and see if the gear lube is foaming, some gear oils FOAM I learned this in my 90 Surburban, I usually use Valvoline gear lube, the store was out so I used Penzoil after 500 miles I stopped at a rest area to take a break and smelled gear lube, their was lube on my rear bumper, the lube foamed at long hwy speeds and was dripping out the vent.
STAY AWAY FROM LUCAS!!!!!!!
Seen that little gear demo thing on the counter of your auto parts store, turn the lucas oil side real fast for 15 min, have your friends help it all turns to foam, the straight oil side does NOT.
http://www.bobistheoilguy.com/images/lucas/lucas.htm
