alt Hacker NewsAutomotive engineer here. One consideration for the diff motor is that it would REALLY sag the diff. I think the only way to go would be a front and rear generator/motor combo in one housing to replace the transmission, powering the rear end with a driveshaft. A direct drive clutch between the front and rear motor/generator would economize driving and assure high power at high speeds.
How about a front wheel mounted drivetrain with the motors built into the wheel hubs?
That would effectively add 4wd to old cars and allow the onboard engine to work normally with an option for eboost/edrive with the only extreme modification being adding the battery and control modules.