A couple of suggestions.
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Make sure the lug nuts are tight on the tires. I had a 98 Windstar that started to shimmy on the highway (~65 mph). I pulled over and found the lug nuts loose on a tire I had changed a few weeks previously.
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Check tire pressure. Are the tires balanced, have they been rotated on a regular basis? It’s possible a weight may have come off a tire.
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How old are the tires, how many miles? I had an 88 Chevy Beretta with performance tires that had about 10k left on the tread, but it started to shimmy at 40-50 mph. New tires fixed that problem.
Hopefully it’s something simple.
Ed B.