I've not been following but have glanced before.

Surely you have checked u-joints, carrier bearings and drive shafts for dings and length, and slip joints for excessive wear.  Have you checked that someone didn't mis-aligned a yoke on a driveshaft?  Also check torque bars for proper tilting of the differentials.  You said the ride height has been checked; I would double check it, and rear end alignment.

Someone mentioned a bad wheel, and you are pursuing the tires.  It may be a harmonic bounce that concentrates two much smaller problems.