Pull the tunnel cover and checkout the driveshaft. I'm thinking the mower has nothing to do with it. The driveshaft is always spinning at engine speed and if it goes out of balance turning the pto off won't help but idling back the engine will. The shaft should go into the bearing at the rear coupler on the hydro pump. If it's short and not going into the bearing the only thing holding it centered is the rag joint. Who knows what one of the PO's did to the driveshaft. Good luck
Went back and re-read the original post- missed the part about a u-joint at the rear. A lot of u-joints aren't made for 3600 rpm. I think that may be where the problem is, or at least another place to look if you can't find anything wrong with the mower.
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