Bummer,It's not a bad u joint. To easy. Really don't think it's a clutch problem either. The sound stops when the clutch is pulled , but that means the input shaft unloads and stops spinning to. The video is a little deceiving. I was trying to recreate the sound by applying some rear brake in gear to simulate a load. The sound of the trans teeth rattling overshadow the cyclic rumble that is best heard in the last 25 seconds of the recording. This rumble is much worse under load and sounds like a bearing running with no lubrication. The sound has me convinced it's a trans shaft bearing. Beginning to wander if this leak has something to do with it. Input shaft bearing perhaps?
No choice now but to pull it off and open the tranny and have a look.