Norman, I would consider replacing the rear lower trailing arm, forward bushing with the insert and poly bushing. It has cured several vibration issues.
firstname.lastname@example.org Contact Dan, as he has new trailing arms and the inserts.
Interesting-the tech who worked on the car mentioned wanting to check that when he did his test drive. I guess it was good. The remaining vibration is speed sensitive and feels rotational. That said, however, I'm going to ask if that inspection was actually done.
After thinking it through a bit more carefully, I believe whatever was the original source of vibration is still there but because the drive-line "anchorage" has been tightened and strengthened, the manner in which the vibration is now experienced has changed. Just for "shitz and giggles" do you know where I can buy the bushing (s) ? Thanks!
Contact Dan —at the email Russ gave you in his last post
Dan makes new control arms with new bushings and the only ones available that will work correctly on the market today in our year Cadillacs
No other options known that will work to repair the rear control arm bushings for our year Cadillacs are available
Dan’s one exceptionally innovative engineer and makes an exceptional replacement that Russ installed in his own car so part has passed the required testing by Russ