Once I got the door opened up (I hate doing that), I checked voltage at the motor connector and had a full 12V. I don't know what the ~8V was about, but apparently the mis-colored wiring diagram in my Haynes book, combined with my usual over-analysis, put me on the wrong path. The engine lugging told me the motor was trying to run, and I still think the motor itself may have been fine. In order to get the old motor and track out, the window has to be lowered enough to get to the bolts holding the glass to the track. Being inoperable, I wound up cutting the steel cables between the motor and the track - which should have allowed the window to move freely. It was jammed tight, I couldn't budge it.
In the process of getting all the old hardware out I destroyed the old motor so it wasn't possible to ever determine whether it was still running - but I'd bet it was.
I think the moving parts of the track had worn to the point where they had enough play in them that the whole thing just got all bound up. With the motor cables cut and the track hardware all loosened from the door, I fiddled with the glass, beat on the door, and cussed at it enough different ways that the glass finally freed up. The rest of it was a piece of cake by comparison, although I did have to figure out how to take the polarized plug apart on the motor connector so I could reverse the contacts; the motor ran backwards relative to the switches, as delivered.
Bottom line, a new motor/track assembly did the trick, even though the motor itself was probably still fine.
