With the drivetrain vibration fixed (sort of), the next problem is that the batteries have been so cold that they just won't give me enough juice. Using a surface thermometer, I measured the temperature at 33 degrees F. Brr.
I ordered battery warmers from KTA Services, one for each battery. Farnam Battery Heater Pads, thin mica sheets sandwiching a ribbon heater. They run on 120V AC, so I wired them so that they will be powered only when the car is charging. For once, I actually successfully planned ahead when I brought a neutral line into the car in addition to the two hot wires and ground. In case anyone's interested, here's the wiring diagram for the heaters. As you can see, I split the power for the heater between the two hot leads of the 220V AC coming into the car.