The vibzzz are killin me!
.............I've fixed a vibe by just rotating the shaft 1/2 turn and re installing it in the rear end yoke...............kim.................
Most likely the joint wasn't seated correctly and it went unnoticed and by changing it and putting it back the 2nd time you did wind up getting the clips seated correctly in the end yoke and not one side of the clip in the end yoke and other on the strap side which does have the ability to rock in the groove of the cap and make a .005 to .010 run out which has a dramatic effect.
Otherwise there is no way possible to change any part of the shafts shortcoming because of the end yoke.
If it has a issue in the end yoke it will have a issue no matter how it is phased to the end yoke.
The ONLY way it could have any orientation issue with a end yoke was if it was built and trued with that end yoke, and that EY has a defect and is out on one side.
The shafts at the time were made by detroit, they weren't making them with EY or balancing them with the EY from the diff, as far as i know they were using a inside clip fixture to hold the weld yokes into the tube for truing.
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I know people say its true, but if it was i could take a shaft swap the orientation on phase end on my yoke fixture and it would balance differently, it does not.
Same would be when i swap end for end and now the shaft is spinning opposite direction, nothing happens.
However dropping a shaft and it hits the floor just right can and does change the balance, setting a joint to one side of it's lock up and not setting the joint to center does change balance, which is very likely to happen in some EY's and these are tests i have done.
This is why very good EY are extremely tight on the joint and need light persuasion to instal the joint into, it's only a few thousandths and the shaft will have a problem
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