I thought about this as was laying in bed waiting on a fat dude to break into the house last night.
The vibe/noise certainly has a strong resonance aspect to it. I'm wondering if it isn't due to the style of Hack n Tap yoke...
The IRO yoke has like 2.5 of spline with a 1/4" of open behind it. Even though it is bolted to the output shaft, I wonder if the yoke is moving up and down at higher speeds?
That would/could explain the severity of the vibe.
It would be easy enough to cut the yoke shorter so it would have less engagement on the output shaft. That *should* allow for less potential for movement and resonance.
[USER=2173]@ghost[/USER] I know you NAXJA geeks 
have good success with the IRO kit. On an XJ application the output shaft is cut shorter to allow use of an XJ front shaft in the rear. This is what got me thinking thisnmay be a solution to my issue.
