You might just get this off of Ebay:
Jaguar Daimler Diff Breather Elbow Tube Fits XJ XJS E Type 420 C25716 8509 | eBay
Uhhh, considering your differential is currently just sitting on the garage floor, now might be the opportune time to undertake the massive job of screwing it in to the diff case.
EDIT:
Terry's Jaguar Parts in MI has it cheaper....