1999 Ford Windstar shaking at 60mph and up

I have a 1999 ford windstar 3.8 with 1700000 miles on it. The van is fine untill it reaches 60mph and up, the rear seats; second row and rear bench starts to shake pretty bad. I replaced all four tires, the rear shocks are kinda of new, i had the tires rebalanced, I rotated the tires and the shake is still in the rear. I also replaced the rear shoes and hardware. All the busihings look good. Can you help. thanks