Welcome to the forum. It's commonly referred to as "the clunk" and is caused by worn rear sway bar bushings. There is a TSB and your dealer will fix it for free. Or you can spend the $25 in parts and fix them yourself.
Here's a thread you should read with common MSP problems and fixes:
http://www.msprotege.com/forum/showthread.php?t=50549
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Here's the specific infor for the bushings:
In case anyone is wondering, here's the full part number and pic of the new rear sway bar bushings ending in 156E. Hopefully I'll have time tonight or tomorrow to install them and let everyone know how easy/hard they are to do yourself. I was quoted 3 different prices ranging from $11.88 - $14.50 each bushing.
EDIT - I replaced the bushing myself last night. Fairly easy process. You only need to:
1) Raise the car on jackstand - If you don't you will not be able to access the bolt on the front side.
2) Take your 14mm Socket wrench and undo the 2 bolts holding in each bushing mounting bracket.
3) Slide off the mounting hardware and remove bushing.
4) Put new bushing on bar.
5) Put mounting piece back on bushing and position correctly.
6) Reattch the 2 bolts for each mounting piece.
7) Lower you car back down and go for a test drive.