windstar shakes while stopped

A couple days ago, my sister told me her 2000 ford windstar was acting funny. I drove it, but didn't find anything unusual until I stopped. When
its in drive, and stopped, it begins to shake. The longer you are stopped, the more it shakes. My question is what could the problem be?
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Sam wrote:

If you put it in neutral while the shaking is going on, does the shaking go away entirely, or only subside a bit?
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