Getting the Pinion to stay on the motor shaft?

[Thirdgen89GTA]

That was the test fit with the spacers underneath the differential. Those are gone now because the chassis was clearanced so the spur didn't bottom out.

Here are two new pics from that. It looks like the pinion slipped a bit before I tightened it down in the opposite orientation to the way I had it before. Thats as far back as it will go with the set screw tight. I'll swap it again.