Assuming the circlips are all installed on the cups, my money is still on the shaft has some known but unquantified runout that has reared its ugly head now that driveshaft speed has gone north of 3000 rpm at cruise.

Also the yoke looks a little worn where it rides in the bushing which likely means the bushing has some excessive wear also. If you grab the yoke and shake it while it's in the trans, you should have to pay attention to feel if there is any slop as in less than .010".

If it has much more than that you should look at a new yoke and tailshaft bushing. If it seems excessive and you have another yoke that doesn't look as used as that one does, slip it in and see if it's tighter.

I went thru this with a Ford top loader after I went from a 2.75 to a 3.50. I changed the tail bushing which helped but my yoke looked like yours and when I swapped it out it drove like a new one after.

Kevin