It's an axle design that is unique to Cadillac and was never installed on any other GM make. Therefore, parts are also unique to this design, and are not interchangable with any other GM rears.
Don't ask me why....... other divisions all were fine with variations on the GM 10-bolt and 12-bolt rears, but Cadillac had to be different.
I suspect the axles are a bit stronger that other GM rears because Cadillac does make some heavy duty chassis for hearses, ambulances, stretch limos, etc. Perhaps GM was concerned that regular car axles would not hold up.