Here's my update...
After 5 days of Nissan driving my car they couldn't locate the exact location of the sound. They tried swapping axles, driveshafts, and other parts but couldn't nail it.
Finally they tightened down all of the suspension components and also greased the axles again (the previous dealer only greased the diff side, not the hub side which was dry). After that the noise went away.
I've had it for about 4 days now and I haven't heard the sound except one time pulling out of my parking spot in the garage. May have been my imagination but we'll see what happens. If it continues they are going to replace the diff, but I'll also have them check the cotter pins as you had suggested.
I've only had this car for 2 months and it's been in the shop for half of that. Sometimes I wish I wouldn't have gotten rid of my Mazdaspeed3. But I did buy a 09 Z so maybe that's part of it...
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