Road Force balanced Tires - Chevy Shakes

[FL335i]

Ok, while I had some days off for Thanksgiving I had my tires RF balanced on a Hunter 9700.  I witnessed the whole process.  Excellent technician which is rare to find on the 9700.  My Ko2's have approx 20k miles on them.  My truck has always had a slight 'Chevy shake' condition in the rear-end in the 76-79mph range.  The shaking is not as bad as others I've seen on Youtube or on this forum.  So I've had the shake with the factory SRA's, then at 53k went to the Ko2's just spun balanced and still had it.  Now at 73k I had my Ko2's RF'd and it did smooth out the truck overall, but it just made the rear-end chevy shake that much more obvious.  The truck now rides smooth as glass up to 75.  Then this arse end vibration pops ups and doesn't go away until 81 and it's still there just less noticeable.  You can tell how smooth the truck can/wants to drive, but there is something just F***ED in the rear end.  I've checked drive-line angles on the U-joints, came up good. I downloaded the 20 page TSB document from GM that outlines the vibration diagnostics and checked my driveshaft for run out, I'm well within spec.  The same vibration has been present on two sets of tires balanced 3 ways.  Same dang rear end vibration.  My next idea was to get a good shop to balance the driveshaft, but after what I felt today on I-4 from Tampa to Orlando and back.... and seeing some posts about the axle shafts fixing it.   I'm pretty sure it's axles.  It feels like the axle shafts are bent or the flanges for the wheel mounding is out of round.  I'm looking at doing Yukon shafts and a Detroit locker.  And maybe spin balance the drive shaft so I can check that box off. Annoying shit how nice these trucks are and GM won't fix it.  I took my early 90's IFS/Coil 4runner and modded it to leafs and and a Dana 44 straight axle up front.  Cross over streering, 37's and locked F&R.   CV rear drive shaft.  That thing would do 100mph smooth.  But my factory built stock truck shakes at 76+. I'm not interested in buying a 2009 Ram with a 2017 window sticker. And the new F150's are pretty dang expensive and i'm not fond of them.  If there was a perfect truck to trade it in on.... i would.  Yukon shafts and a Detroit sound like an option.  I'm building my K5's rear axle the same way in the coming month... Yukon shafts and a Detroit locker.  Already built the front axle my K5 this past year....  It's like a run a axle shop in my garage.  Even my 335i's diff got a Diff build!