Hello and good afternoon. Take a closer look at the source of the leak. See if the fluid is higher than the pinion. There is a bulletin for a fluid leak from the top of the differential at the rear differential vent cap. I had to do this on my CTS-V for also seeing fluid under the rear. The pinion seal may very well not be leaking. The fix is relocating the vent to a higher point via a hose and a new vent. I can provide the bulletin if that happens to be the issue.
Your JA Expert,