[Howlermonkey]

Every single driveshaft I have seen that broke/twisted did so because it was either bent or dented.

Was there any noise when you drove the car to the dyno session?

I would inspect the floor of the car and look to see if whatever equipment that was used to transport or move the car touched the floor which might give you evidence.

 

[Howlermonkey]

I'm drawing a blank here since the rear mainshaft of the tranny getting serious runout at high speed could cause the driveshaft to fail but I'm leaning toward the driveshaft failure causing the tranny damage.

It may have had a stress crack that caused the driveshaft to break enough for it to twist but stay connected (for a couple of revolutions at least) while pivoting on the point that is still connected.

This would cause the driveshaft to impart huge forces on the tranny before it ultimately came apart.

Big enough forces to bend the output shaft.

I really don't see any other way the output shaft could bend like that unless it failed the way I described.

 

[Howlermonkey]

I thought that for a minute but then remembered that the driveshaft beat itself against the tunnel as the rear end and wheels spun down which folded the broken ends of the driveshaft that direction.

Very tough call and still not sure of my hypothesis.

Just glad nobody got hurt.