Investigated the internet, and decided I'd try using hot tap water, Dawn dish soap, and borax for the scouring. I had two gray totes that I marked off in one-gallon increments.
Soaked the first batch for about an hour in cool water to rinse away the worst of the dirt. I should possibly have soaked it longer. I filled one tub with four gallons of hot water, 1/2 cup borax, and eight squirts of the Dawn. It made the water such a pretty shade of blue! Soaked it for 20 minutes, then rinsed it in a second tub of plain, hot water (I filled both tubs at the same time so the water would be approximately the same temperature and not "shock" the wool, which can cause felting).
One bag of wool, ready to be scoured:
Soaking the first batch of fleece:
Soaking in the soap water on the right, clean rinse water on the left:
Did a second batch from the cleaner center of the fleece without the pre-soak.
Last fall I bought a Hamilton Beach "drying station" which is supposed to help sweaters and other hand-washables dry faster. I thought it would make a pretty spiffy wool drying rack, and it seems to be doing an admirable job!
I'm going to experiment tomorrow with using the "washing machine" method of scouring fleece -- I'm going to fill up mesh bags with fleece, fill up the washing machine, add soap, add the bags of fleece, then let them soak in the water (no agitation!), then spin them. Remove bags, fill with water again (no soap), then let them soak to rinse and spin before putting on the drying station to dray.
