LSD - Limited Slip Differential. What? What did you think I meant?

I've been noticing a noise now and then in my rear end, my Bimmer's rear end, usually only at low speed while making a sharp turn.

A few week back, I had the rear up on jack stands for something, and I forget why, but I turned one of the tires and I noticed the one on the other side turned in the same direction. It was in neutral, but my experience in the past had been that the planetary gear in the diff would make the other tire turn the opposite direction, even while in neutral.

So I put it in gear and the tire/wheels would only turn with some difficulty, making a hell of a grating sound. Then I'd put one tire on the pavement and the free wheel would behave the same on turning. Are these things posi-traction, limited slip? I've never had a posi-traction rig before so I'm not familiar with how they work, or behave anyway.

Now I'm wondering if that noise is the posi. breaking loose when it goes around a sharp turn. And worse, is it only a matter of time before this thing goes? I recently changed the oil. It was making the noise before that.

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1986 300SDL, 362K
1984 300D, 138K