CTR/ITR LSD is the same thing. I just got my answer after looking around on HT for about an hour lol. Just in case if anyone else is wondering the same thing, the answer is it is NOT a direct fit/bolt on. The final drive of the 99+ SI S4C tranny is supposedly 4.266 and what is need is a final drive of 4.400. So it can fit with a GSR/ITR final drive ring.