Bit of a daft question maybe but are you still sure that it's your diff and not your slipper?
Glazed slipper plates can make a fairly similar noise to a slipping diff.
To help identify whether it's diff or slipper, I mark a line on the outside edge of the outer slipper plate, hold the left rear wheel and spur gear in one hand and slowly try to move the right hand wheel. If it slips but the line on the outer slipper plate stays still, then it's diff slip but if the line (and thus the slipper plate) moves, I know its the slipper and not the diff.
Once the slipper plates have glazed, you need to replace them. Their Hard anodized coating makes them very unforgiving to repairs. You might get away with some scotch briting but generally, once they've glazed, they need replacing.
It's amazing what a new set of slipper plates feels like
Don't confuse the slipper plates with the pads. The fibre pads can be bought back to life by flipping them or by gently scraping the glazed surface with a scalpel blade but the plates themselves are best replaced.
Just a thought - since most the thread has been discussing the diff and your strip down has not revealed any of the normal culprits, perhaps the issue lays elsewhere.
HTH.