Are my rear wheel bearings shot at 50K?

A quick way to test a wheel bearing is to gind a big parking lot (empty) and do some aggressive circles, first in one direction, then the other. A bad wheel bearing can get noticeably louder on the “outside” of your turn, as the weight of the car shifts to the outer tires.

Otherwise, if you are feeling handy, there are several things you can try. Jack each rear tire up one at a time and spin by hand. See if they sound different, and if you can hear the growl. You will likely hear a bit of scraping from the brake pads too (normal). Also wiggle each tire top to bottom, and side to side, to see if there’s any play while jacked up in the air.

I did have a bad tire once on a previous car that I originally misdiagnosed as a bad bearing (to the point I replaced the bearing/hub assembly, oops). The tire had a weird flat spot in the tread, almost like the tire locked up and shaved a small section flat. If you suspect a tire and you have access to a spare (but you have a Prime), you can swap out one side at a time and see if the sound hoes away. If you don’t have a spare tire, you can try rotating tires front-to-back and see if the sound moves. I was lucky enough to have two full sets of tires, one winter, one summer. I swapped the two rear tires and the sound was gone instantly. Once the suspect tire was placed on a balancing machine, the flat spot was obvious. This may not be your issue though. Just another possibility that may not be a bearing.