Driveline vibration

[DWC]

Hoping the collective wisdom here may be able help me out.

My daily driver is an 05' Silverado SB. Wanting to improve it's "attitude" I installed a McGaughy's 4-6 drop kit. Spindles, FR springs, RR flip kit. Ever since the install I've got a vibration/shake coming from the RR that I can't seem to fix. I swear you can feel it just going down my driveway but it gets progressively worse all the way to 50-60 MPH. It seems to come from the rear and move to the front then back to the rear. In sort of a wave if that makes sense.

Fixes I've tried so far:

Swapped the current rear tires for the rears from my Burb, they bolt up but look weird. Not much better, the Burb has much newer tires so that may be the difference.

Checked the pinion angle. Installed a 2 deg. shim in the rear, it is now +5 deg and the engine/trans is -4/5 deg. They are within a degree of each other so this should be OK, right?

Loosened and re-tightened everything I touched with a wrench.

In the half dozen trucks I've lowered I've never had something as frustrating as this vibration.

Any advice, opinions, past experience, anything would be appreciated.

BTW, it does look much better....