rear brake suddenly screeching like Tarzan

BOOM there's your problem.

You un-bedded your brakes. Before I read this, I was going to post that you should do nothing, that your situation sounds exactly like any of my bikes after I have been out riding in the rain, because water splooshing over and onto the rotors removes the bedded pad compound that is invisibly coating your rotors. So for at least a short time you get screeching and vibration as the pads run against the bare metal.

Solution is to bed your brakes. If you live in steep hills like I do, the screeching and vibration goes away by the time you get to the bottom of the first hill. But if you are doing more gentle braking on say flat land, it can take a while to go away in normal use because you aren't getting the pads up to high temp.

So... do this.

I forgot to add that I followed this procedure, but it took only 100 m of the very steep downhill road near my house to fix the problem...