what would cause a "grinding" sound from the rear that oscilates in pitch.

Wheel Bearing or Hubs. I would check them out. Sounds exactly what you are describing (sound wise, that is).

Thank you! I was avoiding that option as the bearings are press on. But you have confirmed my fear, sooooooo.......but, really, thank you!

I was happy to help however, I am sorry to be the bearer of bad news, I wish I could assist you in repairing them. In that vehicle I would pull some wheel bearings/hubs from a wrecked or salvaged Vibe and you should be able to get them at a fraction of the cost. While the wheel hub is an intricate piece as with everything these days, there is a YouTube video to walk you through the steps, or if you are old school (like me) write down what you take off as you go. Its helpful to pull it from the wreckage yard yourself because it sort of teaches you how to take the old one off your current vehicle and what it will look like to replace it. I hope this helps and best of luck!