The verdict from my mechanic: there was a hole in the steering box, and the power-steering fluid had all leaked away. It apparently happened so gradually that I didn't realize I was losing the power steering. (The van still drives pretty easily even without any power steering fluid.) Parts and labor to replace it: about $560, and I can come get my van tomorrow morning. Under the circumstances, that sounds fairly reasonable. As usual, my mechanic (Fremont Wheel & Brake in Fremont) does right by me, which is why I keep taking my vehicle to him even though it's now much less convenient to do so, involving rising earlier than usual, driving to Fremont, then taking the train to work and then taking the train back to Fremont later to retrieve it.