My 2000 626 4 cylinder also clunks from the rear when going over bumps. It happens regularly every 40k miles after being fixed. Each fix costs $250 and they have replaced: rear stabilizer bushings, link control stabilizer, stabilizer bracket. I have been in for a fix 5 times in the past 140k miles. I think this is a design flaw from Mazda but my dealer says normal wear.