So, stepping forward and back. The rear axle vibration is perfectly described here:http://www.vibratesoftware.com/html_hel ... Joints.htm
A third order vibration caused by excessive driveline angle. Three bumps for each revolution as the joint hits it's limits. Well, under acceleration the rear squats, increasing the angles. I measure the half shaft angle now as 4 degrees UP bubble at rest. That's right, tranny is now lower than the axle. With 5 degrees being the maximum working angle of a CV joint, that'd be rather likely to be exceeded under load.
I got some rubber "dog bone" style spacers to test the loose spring theory up front, and a quick insertion improved the knocking springs, but hurt the ride quality. Time to try shims between the upper spring perch and top nut. This'll lower the front a tad more but preserve the most travel.
Looks like I need to raise the rear to remedy the driveline angle. Will test with the rubber spacers, then decide whether to go all the way back to stock height or use a doughnut-style spacer to just add back 1/2 or 1". I won't mind raising the rear, as the rear fender gaps are an inch smaller than up front. The front, I still want low. Even with clicking springs, cornering is greatly improved. Rain finally returned to Seattle after a 3-day drought, and the understeer is definitely reduced. It wasn't just transient traction caused by dry pavement!
Lastly, I prefer the feel of progressive springs. Smaller bumps are smoother and the car no longer crashes off speed bumps or into potholes. Rebound is more energetic though, will study on shocks more once the spring situation is settled.