Update: Went to my local Napa today (with the original rear u-joint and a new replacement unit in-hand) and we compared about every remotely similar u-joint they had on the shelf. No luck. They called their driveline shop that they've used forever and discussed with them. They said it sounds like the yoke got sprung inwards both sides. If I bring it to them they could spring it out. I already knew it was not bent or sprung etc. because I've been measuring the heck out of it. Either way had to get to work.
On the way to work (stopped at a red light) I looked around for a decent driveline shop. I found "You Can't Beat My Shaft!" - Welcome
. It looked like just what I needed. At lunch I went over there and told my sad story. At first Bill thought I may have the wrong unit, but then figured out that I just need to squeeze it down to fit as the grease seals are designed to compress down just for this purpose. He showed me in a yoke he had there. I measured the yoke and it was 65mm like mine. I super verified that he believes the seals will be happy like this indefinitely and he said yes. He said it looks like a new design of grease seal so perhaps that'* why it'* hard to compress.
I was still skeptical.
After work I went home, ate a bite, crawled under there, and tried to compress it with the super mega robot strength in my hands. Surprisingly this did not work. I put one side of the u-joint in the yoke then used a c-clamp to compress it and then was able to push it in place like that'* how it was meant to be done. Strapped it in and drove it about ten miles at around 60MPH. Crawled under and it was cool as a cucumber on all four caps.
Success? Woohoo? I'll change the question marks to exclamation points after I drive a bunch tomorrow.
Regarding the unit that the GM parts counter showed me: those bronze-looking rings apparently push inwards with a bunch of force thereby allowing them to fit. I wouldn't have guessed that because those looked and felt solid . . . but now I look at the pictures and see that that surface of the crossbar is machined quite nicely.
I hope this experience helps someone out there. Sure was a struggle for a simple solution for me.