possible drive line vibration / shimmy?

[itscoo2pyopants]

I have been having this vibration / shimmy at highway speeds (60-80mph). the weird thing is that it is intermittent. I feel the vibration underneath the drivers seat, and I feel it in the steering wheel. It will shake for approx 10-15 seconds at highway speeds, then drives smooth for 10-15 seconds, then the vibration comes back. throttle doesnt seem to affect the symptoms. It seems to be worse when the vehicle is cold, gets better as the day goes on. I am at 170k miles, but this has been going on for at least 10k miles. seems to be getting worse

things ive checked:

rebalanced wheels / tires. tried a different set of known good wheels tires. removed wheel spacers, no change

new front axles, wheel bearings, hubs, front suspension replaced within the last 10k miles. alignment done as well. does not pull and tracks straight.

rear suspension bushings having approx 20k miles on them

bilstein struts / shocks with 50k miles on them

underneath the vehicle nothing "obvious" stands out. the only thing ive noticed is the front guibo has slight cracks, no chunks missing. im leaning towards replacing the guibos and center support bearing to see if that'll fix this issue. should i replace the rear CV joint "while im in there"?

any help would be appreciated