I have been hearing a rotational mild clunk or click in the rear and most predominant at around 40 mph. I put the car on jacks and put it in neutral to try and demonstrate and replicate. Video link is below. Any thoughts?

 

Wheel bearing?

 

UJ clacking exhaust ?

 

UJ is a possibility, but the UJ web bearings aren't rotating at that speed

 

My guess is wheel bearings or spider gears in the differential.

Get under the car and put your ear close to the differential while a helper does the spinning. Is the noise louder at the diff?

The noise is rather loud. If a wheel bearing, it's pretty well shot, and I'd think you should be able to feel some roughness right at the hub.

Wheel bearing failures and diff failures are not uncommon on X300s

Disconnecting the half-shaft would help isolate the problem but doing so isn't a 5 minute thing

Cheers
DD