The rear clicking noise happened on my car (84 300cd) also. I found out I had three problems Here they are (were) ranging from the expensive to the cheap:

1 The left rear axle shaft (including the CV joint, which was making the noise) was toast. Torn boot and all.

2 The driveshaft bushing? insulator? big washer? Okay, it's a black donut-looking thing that mounts between the differential and the drive shaft - I can't remember what it's called, but mine was torn and flopping around a bit while the driveshaft spun.

3 The rear sway bar mounts. Plastic ball and socket things that hold the sway bar mounts in place. one of mine was broken, causing a random rattle at parking lot speeds.

I had all three of these things fixed and now no rattles .

Hope this helps.
-Gil