I was going to say the same about the additive - I bet you it was that which caused the noise etc.
I had the same thing with an Isuzu Trooper a couple of years back - they can go with a hell of a noise when you try to turn, even going slow.
I think what happens is the diff. stays put and the tyres flex to take the strain until it can't hold anymore and then....CLANG! it resonates down the prop. shaft.