Shaking/vibration, no noise?

The vehicle listed above is an EL Limited.
Roughly six months ago I started chasing an intermittent drivetrain shake/vibration, but it's never made a sound which has me stumped. So far, I've replaced all brake rotors and pads, vacuum pump (and lines, "filter", and 4WD solenoid), IWEs, front CV axles, front driveshaft (xfer case to front diff), front lower ball joints, rear upper control arms, and all four coil overs. Had a local shop that I trust check it over twice, then perform a balance and alignment, replace all drivetrain fluids, brake fluid, and add a shudder stop additive to the tranny. Frustratingly, everything but the vacuum pump was worn or had failed in some way shape or form and actually needed replacing. Checked rear driveshaft when I replaced the front, u-joints seemed tight. No clunking or excessive play in the rear CV axles. Every time I've had the wheels off, I try to test the wheel bearings out of pure paranoia, they all seem fine with no play and no noise, though it's hard to fully test them without fully removing them or pulling the CV axles. Tires are new as of 12 months ago. Through all of this, there's never been any sound whatsoever associated with the shaking.