Take a look at the rear diff, there should be a data tag on the housing left and above the fill/check plug. That will show the build date, if is is a 2012 date is is original, a later build date it has been changed. The only way to check is to pill the diff and check the nut. 20% stay tight, 80% are prone to coming loose and fail.
Mileage or driving style doesn't appear to be a determining factor. We have seen them loose, <15ft,lbs at under 400 miles, the other end of the chart >8000 miles and still tight.
Kind of like rolling the dice, some rolls are winners some rolls crap out.