Someone on the AEVA forums had suggested that the vibration may be resonance in the motor mounting system. I had my heart set on this being the cause - alas no. I'd even measured up for motor cycle steering dampers.
Placing a jack with a block of wood under the centre of the motor frame very marginally decreased the vibration is accord with what you would expect in that it offered a bit of dampening to the real cause. I used my phone to record the subsonic audio coming from the car during axle stand testing. Here is a spectrum plot of the recorded audio with speed somewhere between 55 and 60 km/h.
It still looks like interplay between the UJs and the motor/mounting.
Crawling around under the car travelling at 55-60 km/h (on stands) the vibration was almost non-existant at the diff end. The motor itself was the worst point. I could not visually see any runout consistant with this behaviour. I've had some really bad UJs in tailshafts in my life that didn't vibrate as badly as this apparently OK one.
Precision Balancing have a portable setup for analysing vibration that they use in the field - it may soon come to that.
I'm also thinking about CVs....