We took Sophie to the vet. The prescribed arthritis medication and glucosamine marked a drastic change in her temperament. Until it didn't. The effect lasted about a week and a half. She had stopped lunging at random cars, and confined her crankiness (lunging, teeth baring, ferocious barking) to mail carriers and all delivery folk. Then she started again. She's also limping, so maybe there is more going on than just arthritis.
The vet had recommended we talk Sophie to a vet who is also a certified animal behaviorist. The price tag for this is rather high, but the vet told me she thought it would be a worthwhile investment.