The clevis turning sounds similar to what I described above. With the spacers in between the clevis it takes the slop out of the hitch so it cant twist as much. I have never seen anything twist as much as you are talking about. Does this mower only have one tail wheel? If it does that would really let it rock. If so I would definitely go to two, especially with an 8 footer.
I dont see how that twisting would hurt the u-joints. The twisting is in the same axis as the PTO. Those joints will take a beating when turning rear tight. My 12' mower has a c.v. joint in the PTO plus a single u-joint at the mower, with that you can turn it 90 degrees without any PTO binding. The rear tire will hit the tongue first.
Let us know how it works.