Hey! I am new here! Also proud owner of 91' sierra with BOA engine and Mt75 gearbox.

But since I bought it I am trying to find issue and how to fix it!
My problem is... Everytime I take off or trying to fast shift i hear that KNOCKING sound from behind!
Its like my axles want to break or diff have HUGHE loosenes!

So I started to change everything, I replaced axles bearings inside and outside - same
Then differential....
Same..
SO I thought... maybe that rubber thing between gearbox and driveshaft is gone? - well... same

Turned out that I can wiggle my end shaft? That one which comes out from gearbox. I can turn it left and right a LOT...
How to fix this thing? My dad told me that I can use some kind of shims, to remove that looseness.

Any ideas ?\

Its like on this video... But MORE!

 

That play is normal bud. Check your engine mounts and diff mount.