8.75 diff - metal in oil (pictures). Is it ruined?

With junk in the bearings, the preload measurement will not be accurate, and probably not the backlash either.
As said above mark at least one cap. I do this by center punching the cap and case adjacent to each other, about 3/8 inch on either side of the parting line. If you mix them up, they will often not take the adjusters back in. Exactly where the marks go is not important. What is important is that at least one side is so marked.
Don't worry too much about putting it back together; there are several guys here that can help you through the reassy.
If you find that you have a Trac-Loc, Check down in the holes where the axles go, and make sure the thrust-buttons are still in there;one on each side.
Also check your axle bearings, and inner seals.