I think I've found a solution, though I don't know why it works. I'm new to this office and many of these links were made before I arrived. By going back and relinking B's file path within A, I was able to see it in D.

What's puzzling to me is why that faulty file path works in A itself. That is, when I open A, the faulty xref file path brings up B without a problem. But when the faulty path to B gets nested within A, it no longer works.

I'm hoping the relinking I've done doesn't cause problems for others on the server; this may be an untenable solution in the long run, but at least it works for now. Thanks for the replies, all. I'll keep you posted.