Do you have a feel for how much of the driveshaft rotation is caused directly by rotating the rear wheel? Or maybe a better way to ask is how far you must move the wheel before you see the drive shaft react? If you put one hand on the pinion yoke at the rear U-joint and the other hand on the rear wheel and work them back and forth you should be able to feel how tightly coupled they are. Any difference between rear wheel rotation and drive shaft rotation is due to gear lash in the rear differential and its normal to have some small amount, but not as much as your video. Unfortunately I cant tell you exactly how much is acceptable but an experienced drivetrain mechanic should be able to assess it and there might be a spec someplace although I couldn't find it.
You might also be able to assess the 2 u-joints by putting hands or tools on either side of each U-joint and working them back and forth while looking and feeling for any play or odd movement that isn't in the direction of rotation. U-joints should be extremely tight and have almost no detectable play in them. If there is play within the u-joint then you are on borrowed time and it could be dangerous at speed, although they do usually vibrate severely just before they completely fail and fly apart. They can go from loose to the fail point pretty quickly but likely the extreme noise and vibration would make you pull over pretty quickly
If the rear differential, u-joints, and flex disk are seemingly in good shape and most of the play is in the transmission it might or might not be OK to keep driving. I think the best way to judge is by the amount of noise that you discern is coming from the drive train in various gears, at various speeds, under acceleration, downshifting, or coasting. Your posts so far describe static testing in the garage but its good to go out on a quiet morning with the windows down and really concentrate on what noises you hear while moving at different speeds. With experience and some thought while looking at the drivetrain diagram you can logically deduce where a problem might be based on when a noise is made and what stresses are on the drive train at various speeds and in what gears, or coasting etc. There certainly bearings and gears to wear out inside the transmission but you rarely hear of failure without warning.
In the end, if you hear much of anything from the drivetrain at any speed its best to just take it to a mechanic and let them diagnose it if he knows his way around a pagoda, but even if not, you could take any good mechanic for a ride and let him listen. Likely they will tell you how urgent the problem might be. Almost every RWD car out there has similar issues and fail modes in the drivetrain because they all have gears, shafts, and bearings. Most large cities have a drive train shop that specializes in repairing drive shafts, u-joints, and the like. If you try any work yourself be aware that driveshafts have to be balanced at speed when replaced or repaired just like your wheels when you put new tires on them. You need to pay special attention to orientations so you can replace things the way they were. I marked a reference paint dot on each part of the drivetrain before I rebuilt it all. It's not that hard of a job and there's some good help on this site.
Lastly, I see in the picture you sent that the upper differential trunk mount has completely failed. There should be a gap between the plates. There are pictures on the site. This can make some really amazing sounds, especially while downshifting. If its bad then quite possibly your "clunk" bushing is bad too. It's the bushing the two halves of the rear axel pivot on. I dont think these issues are related to the noise in your video , but something that needs service...and unless you know other bushings and subframe mounts have been replaced in recent history, you may find yourself learning just how many bits of rubber there really are on these pagodas. Many of us know where every one of them is. But thats part of the fun. Good Luck!
