I'm no expert, so take this with a grain of salt, but it could be the rear axle spline lube. Going by what I have read on this site, it a fairly common problem. I read someone's explanation of the fix (re-grease the splines) and it sounded like an easy job for most DIYers, but I've never done it. Someone who can be more help should chime in soon. Or search the site. I have better luck using Google/DuckDuckGo/&c and adding "site:the370z.com" to my search string; the site's built-in search doesn't work well for me.
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