[UPDATE!]
Dear Peter and friends,
My Kelley backhoe has been delivered back to me from the Kelley/Deere dealer with all new "updated" parts installed.
In short, it now works like a dream.
Apparently, the backhoe hydraulics was installed wrong when the backhoe was assembled. There is no longer any "surge." This "surge" is what was stressing the parts. I have yet to find out if that was a factory incident or a dealer incident. I may never find out. But the dealer was very accomodating.
The 3-point setup feels much better now. No jerky movements whatsoever. One almost forgets there is a Kubota in front of the backhoe while operating the hoe now. All the stress seems to be directly on the stabilizer legs (as it ought to be.)
I did talk to a nearby Kubota dealer about the reinforcement parts Peter talked about. They seem to think the 7500 has "reinforcement parts" on it already(!?). And to reinforce it further, a subframe is the only alternative (They think I don't need it). Since this Kubota dealer sells almost exclusively to wine growers, perhaps they may not be the best authority on backhoes.
[I bought the new Kelley backhoe from a John Deere dealer because it was inexpensive ($4800), in stock, and the people there were nice to me. The dozen or so Kubota dealers I contacted wanted $6500 up, and did not have one in stock.]
I shall continue to learn more about reinforcing tractors/hoes as everybody suggested and report back if something "eventful" happens.
Thanks for all your help and this is a great website!