Shaun_95

I have a new to me 95 K1500 that is making some strange sounds, I beleive from the rear end. Main one I hear is a rotational sound which I beleive to be a wheel bearing, not a big deal I can fix that, I also changed u joints as they were original (life time lub yeah right) they were stiff and on there way out. The other thing I have come across is when I back out of the drive way I hear a bang out of one side or another. I had a wonder around the truck grabbed ahold of the top of the front wheel and rocked it back and forth real hard, no slop no noise. Next grabbed the top of the rear wheel and low and behold the bang happened, same thing other side. Now what it feels like or seems like is that the axle is acyually moving in and out of the housing, it slams itself back in making the bang, it feels kind of like it is spring loaded in there. I have looked in my haynes manual but it does not have an exploded view of the rear diff to see what's in there. Is this movement of the axle right? if not what am I looking for when I pull the diff cover, also if I replace the carrier bearings while I'm doing the wheel bearing do I need to check back lash? I have read quite a few posts on here about diffs and have seen to mark what comes out from each carrier bearing and make sure that exactly the same thing goes back in. Any help would be appreciated, never owned a chevy before so this is all new to me. And what is this gm additive that I read about that must be added to the gear oil? Truck is bone stock with 225000 Kms K1500 4x4.

Thanks for the help
Shaun