Could be the yoke or the PTO shaft. If you have more than one shaft, you could swap and see if they all fit the same. Also check for slop, radial and side-to-side in the u-joints.From what I could tell, the movement comes from the shaft fitting a little loose on the tractor PTO (maybe mower pto too). The locking pin is fully engaged, but the shaft doesn't seem to fit supper tight, at least not like a car driveshaft. So could it be the wobble I see might be normal movement?
From your description, I think your driveshaft is ok.