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!