Update
I pulled the IRO yoke off to cut it shorter. While it was off I decided to double check the depth of the hole I drilled and tapped in the output shaft. All good. Said why not chase the threads while it's out. Then I broke the freaking tap off in the shaft.
Chiseled it into pieces but had to cut 3/4" off the shaft to get to where the broken tap pieces were. Drilled out to 25/64 and installed a fresh Heli Coil.
Cut the yoke about 2" shorter and buttoned it up...
No annoying resonance remains. There are slight vibes from 55-58 but nothing I, er my wife, can't deal with.
It seems the Iron Rock yoke was loose enough on the shaft, side to side, and was moving on the shaft under certain driveline loads.
Weird, but fixed now!
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