I have worked on a few 280Zs with rearend knocks over the last 14 years. As you know, these cars are known for their knocks and there are lots of places (both drivetrain and suspension) that can have slop which will cause knocking. Some not often discussed are the rear shock insulators. While these can look solid, they may partially separate. A couple weeks ago, both the left and right insulators on my '76 280s completely separated (incredibly within 2 days of each other while the car was on jack stands) and the wheels slumped down. I had inspected the insulators carefully when I replaced the shock struts. I have now replaced the insulators with used ones, as new are not available, in the hopes that they are sufficiently solid. I will make an entry after I drive the vehicle which will be a few months away. My post is for those who have tried everything else. Good luck to all!