2000 Ford Windstar

Engine Mechanical problem
2000 Ford Windstar 6 cyl Two Wheel Drive Automatic

i have a 2000 ford windstar, 3.8 L that the engine vibrates between the speed of 65-70 mph, only in between that speed the engine vibrates, I recently did alignment/balance on all tires, change tranmission, spark cables, and intake gasket manifold, does anybody know what could be wrong with my minivan?
November 3, 2009.

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