The 4020PS with the under-seat PTO, which is my last, last tractor I am keeping, was in good shape for the bolts. However, the other tractor that was supposed to be my last one had evidence that someone had been in there. They installed the nuts and used the safety wire washers for the nuts. There were, I think, two loose nuts. Putting safety wire washers on the nuts cannot work when you do not use anything on the head of the bolt. Luckily, nothing bad happened. I caught it before it did.
I did weld some on the cage. It was hard to get a decent weld on it, though. No matter how much I tried to get all the oil out of where the two halves meet, the heat from the weld would pull oil into the weld I was trying to get on it. It may have just been me not doing something right. My welding history was shop class in 1978, and then getting a MIG welder years later, and then getting good enough welds on things with that welder.
I have to look at the shims on the carrier again because I have long wondered if the flanged axle bushing could be the cause of the issue if the shims turn against the flange on the bushings. The other reason that I think could be the reason is that sometimes a tractor would come through my hands that made quite a bit of slope noise when you wiggled the rear of the tractor side to side, and that means some have more slope than others. I am aware that the gap must be within a certain range, and adverse consequences can occur when it falls outside this range. So, if the gap is too big and the uneven torque is applied to one side of the tractor, I could imagine that the carrier twisting and getting bound up until something fails.
working with me this summer I have been able to take on work I normally wouldn't , but that makes me even busier .
I can’t wait to see your next loader project. 
Harry