After two "diff hot" messages on a road trip, on my next service the dealer took a looksee and found a leaky seal. Scoped inside the diff and it looked okay, but upon changing the seal, the fluid down in the bottom had more metal than they liked... and after it was back together they noticed more noise than they liked. So they conferred with GM and had the whole diff replaced under warranty.
Thank you, St. Claire Cadillac. Thank you, Cadillac. No drama, it's just fixed.