What kind of tires do you have? I had Goodyears on my Caravan and after about 20,000 miles or so, the ride got incredibly harsh. The tires were rotated 4 times a year and there was no uneven wear. I switched to Continental tires and what a difference. Very smooth and very very quiet. It may also be possible that one of your tires has a broken belt, very dangerous.
Good luck.