Ford Expedition

I have a 98 Ford Expedition, 5.4, 4WD, auto with 170000. It developed a clunk about 2 months ago when you first start accelerating - after about 100 feet or so between 10-15 mph. You can feel it in the floor board a little bit. Here's the catch - it only happens in 2WD, when I put it in AWD it goes away. Sometime one clunk sometimes 2 or 3. I've replaced the left ball joint, shocks, checked the axle, sway bar brace, CV joint. The car has been gone over by 3 mechanics and they can't find it. Help!
May 22, 2007.

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