An update, for all of you chasing this problem, think it's safe to say nobody has spent more time/money on trying to fix this.
Attempts to fix:
Aftermarket drive shaft
Yukon axels
Yukon lsd
New ring and pinions (3 sets)
4 different shocks (stock, belltech, mcgaughys, qa1 adjustables)
3 sets of wheels and tires (two factory, one forged aftermarket)
Every set were balanced at least three times
Leaf spring clamps
Brake rotors on all four corners, twice in the rear
Full conversion to 4 link rear suspension
Solution: none of this worked
Problem still exists in the 65-70 range, I haven't put the EVA back on it since I did the four link but before the frequency on the vibration indicated that the vibration was a wheel speed vibration so potential culprits had to be wheels, tires, axles, carrier, spindles, brakes, anything that rotates at wheel speed. For that reason I didn't expect the four link to resolve the issue, other than maybe the added crossmember might alleviate any frame beaming, which isn't actually an issue based on the vibration frequency.
Since the rear conversion I'm now going to spend some more time focusing on the front, make sure nothing is out of round up there. Still convinced there is a silver bullet that will put this to rest, dammed if I won't find it.
Good luck everyone!