1997 Plymouth Voyager Side to side swaying or vibratiions

Recently while driving at low or high speeds the van has developed a rather strange tendency to shake badly. It seems get much worse when you go over a bum and especially more worse if you go over a bump and are going down hill. When it happens it starts to shake from side to side and the vibration is much more noticeable in the seat. You get a bit of vibration in the steering wheel much not much, its mostly in the seat area is where you really feel it. I did notice that if I steer to the right a bit it will diminish and sometimes stop. Applying the brakes has no effect on it. It almost feels like the body is loose from its mounts! I replaced one tire with a used one from an auto wrecker just before this started, tire was balanced, is a radial, but not the same brand as the other three.)

So far I have not come up with a plausible reason, but am leaning towards the cause being a loose ar worn inner and outer tire rod in combination with worn front struts. Anyone have nay ideas? Or suggestions, would be appreciated.