[MyFirstZ]

The Pinion seal can definitely be done your self. Just make sure it goes in square. I did mine not to long ago you also have a breather at the top of the pumpkin. So see if you can trace any gear oil up to it.

I actually experienced the same noise coming from my ten bolt. I have heard from two different GM techs a few things. One it was normal. Second the C-clips for the axles wear out.

Maybe because my car is freaking loud I cant hear it now but before when i had the magnaflows I never heard that actually causing any type of noise while driving.

I would recommend replacing the c-clips and change the diff fluid. While your in there check for any wear on the axles or on the gears.

As far as additive GM sells a little 3oz or something like that bottle for 10$, then you need two quarts of either tGM's synthetic fluid (27$) or you can use mobil one. (75w-90) I know people that have used royal purple also. As far as filling, normally I will fill until it comes out the fill hole.

As far as knocking over bumps, when you shook the wheel you grabbed at 12 and 6, 3 and 9?
You may not always hear the noise but will feel any play. I know the rear shocks could make a hell of a noise when they start going bad.