Ford Windstar

I have a 99 windstar with 3.8 ltr engine, auto, OD. At city driving speeds, it will frequently shake in the front end. It feels like I am driving over washboard. It doesn't seem to happen at highway speeds. When this shaking happens, I let off the gas and it goes away. It seems to happen mostly when going up hill, especially after hitting a dip or bump. I have been under the car looking for loose ball joints, sway bar, tie rods etc. CV joints don't seem to have excess play. When I bounce the front end, the struts seem to dampen properly. If someone has any insight into this problem, I would apreciate some advice.
July 2, 2007.

How many miles? Over 50,000? Struts, ball joints.

Jul 3, 2007.