If the transmission runs fine just use it. If the transmission is bucking, jerking, or making any grinding noises then I suggest getting immediate help. The piece could have come from from anything inside the transmission. Just note that the average life of the Windstar transmission is about 120,000-130,000 miles. I'd say keep driving it until you find any symptoms. If your really paranoid, bring the hunk of metal to a transmission shop, possibly bring the van too. If that's the only hunk of metal that broke off (you can only hope) you may be fine.
I really wish the pictures were clear, they are very blurry. That can also help us help you.
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1998 Ford Windstar GL 3.8L 253,100 miles
2005 Toyota Prius Package 6 1.5L 135,000 miles
2006 Toyota Sienna LE 3.3L 36,000 miles
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