I remember reading lit on the 97 pre production Cateras that they were going to offer one as an option. I just don't think they ever made it. Are you considering on increasing your engine output? I know they offered one for the european market, or if you are thinking about putting down so big power you might want to find a wrecked GTO and get the rear end out of it.
When I picked up my '00 at the Cadillac dealership, I asked the same question. The sales weasel told that '00 was the last for the "posi" rearend and the '01 had the limited slip rearend. Hope this helps.