Just a follow-up to this problem... now solved.

I did a review of the rear end suspension and could find nothing wrong. Next, I began 'loading' the four corners of the car by bouncing them up and down as hard as possible and discovered that the noise was in fact coming from the FRONT right end, where I had recently done a major rebuild of the suspension. The noise could only be repeated after loading the front LEFT corner, then bouncing on the front RIGHT corner.

Further inspection found that the top nut holding the torsion bar link was loose... not very loose but certainly not to torque. Maybe I forgot to give this a final wrenching but after tightening to spec the noise was eliminated. I've driven the car for a month now and still no noise.

I've learned now that noises that appear to be coming from the rear end of the car may in fact be coming from the front. I won't forget this experience!