Low vibration

I have a 2010 tundra, 2wd, 186,000 miles. It has started making a low vibration and sound beginning around 30mph. As speed increases it starts sounding like I’m running mud tires on the back. It’s quite loud at highway speeds, but doesn’t sound “mechanical”, ie no grinding, no loud whining, etc. Any ideas where to start? I’m thinking tires at first, but both back tires are less than a year old. Possibly the carrier bearing? It is a feeling in your feet and butt as well as a sound.