Update for others who may experience this issue.
I tried shimming the rear end down and it made no difference.
I took the shims out and tried a different yoke.

Success!
I can’t believe it, just got back from a vibe free drive and it felt great.
I changed out the yoke with a new u-joint and it worked.
Luckily I had the original yoke from when I pulled the 518 from the junk yard. It was rusted and scarred up, but I dressed it up.
Worst case scenario is it may leak, but I’ll worry about that later.
Another member gave me a tip about the difference between the 2 yokes. I had never seen that mentioned elsewhere.
Apparently the 518 yoke has a slightly different pitch on the splines compared to the 727 yoke,