the Noisiest Rear End around...third seat?

I dig my "new" 1995 E320 Wagon but have 2 separate noise issues in the rear.
First, there is a creaking noise from the shocks. This is at low speeds, when turning a corner, when backing out of the driveway, etc. Annoying, yes, but tolerable if necessary.
The second one is driving me crazy. When I hit any kind of ripple in the road, a crack in the pavement, whatever, I get a thud from the passenger rear. At first I thought that something was loose in the cargo hold so I went back and checked everything: tools, jack, spare, all seem secure. As does the hatch: it is closing and locking down firmly.
My mechanic said that he cannot find anything with the shocks or the SLS system that is causing the noise.
I do have the third seat option, and I'm wondering if that's the cause. I can't seem to lock it down even though I see a keylock underneath the pull-up handle. How is this supposed to work? When I tinker with it, I find that it's actually hinged and pops up???
Just some guidance on how the lock-down is supposed to work would be great. Or if there is something else that is causing the noise?

[jcyuhn]

The creaking could be the body bushings for the sway bar. They dry out over time. Every time the suspension compresses or extends, the sway bar rotates in the bushings. Because the bushings are bolted directly to the body, any noise they make is quite effectively transferred into the car. If the creaking ceases on a rainy day, try replacing these bushings. They are inexpensive and very easy to replace.

No clue on the thud.

- JimY