It's possible that the axle/bearing is not the problem but that seems to be the most common cause of "rear end clunk" on the 370Z. Seems to be a pretty easy job if you have some mechanical aptitude/experience and the right tools. I'd try that before tearing into the differential.
Sound travels well in metal parts, making it difficult to pinpoint the source at times. A
mechanic's stethoscope will help.