Dodge: Plymouth Voyager steering, plymouth voyager, 1993 plymouth voyager

Question
1993 Plymouth Voyager, 2.5 L , auto. Van has 260 000 km on it. The front end is good and tight and has no obvious parts worn. New struts in last two years and are firm and not leaking. 30 000 km on 130 000 km rated tires , rotated every 10 000 km. Rims OK as well. The van tires wobble under load at about 20 - 40 km per hour. The wheels have been aligned and checked over by two shops ( the original tire installer re balanced the wheels ) and can find no problem. This shaking is quite bad at these speeds , Any idea's what might be causing this. You don't feel it in the steering wheel as bad as you feel it in the seat of your pants and seat, but it is present in the steering wheel.

Answer
Two other things come to mind.
1. Bad front bearings,
2. steering colume bad.
Sometimes, a tire can be good then it could be out of round. What I would do is put the front tires in the back, and rear to the front. Doesnt hurt to see if this would be the cause to.. Thanks,,