Vibration (rear?) on my Dodge Grand Caravan 1996

Here is the whole story: Bought 2 new drums (made in China) in Murray's store. After installation, when braking, noticed vibration. Checked drums and both
were way out of round. Had to drive van with "new" drums for a week before I got good ones. Just thought I fixed everything, vibration started again. But this time I wasn"t braking at all ... Vibration starts around 55 MPH although I can feel that something is wrong around 45 MPH. I took it to the dealer and mechanic told that rims are bent. Bought 4 new tires and 4 new rims, did alignment after replacing rims and tires. Took it to the dealer, again, and mechanic said that new tires are bad. On my comment that there is no change in vibration with new/old rims/tires he insisted that new tires are bad. Mechanic, also, managed to turn ON "check engine" light! His comment was: did you know that "service engine soon" light is ON?!?!?! Unbelievable! Solved that problem by myself. Took it to the DiscountTires and they told me that tires/rims are good.
I can feel vibration on: 1. steering wheel, (no wobble, just vibration), 2. on seats and 3. whole body. If I sit on the rear seat it looks like vibration comes from front, when I sit in driver or passenger seat, it looks like vibration comes from he back. Got somewhere on the internet that there is TBA 22-03-99 (which might relate to this problem) but I was not able to find the whole text. I bought this minivan used at 99640 miles. Now I have 173940 miles. This is the only problem I had since I bought this minivan. There is no problem with CV, transmission, half-shafts, or bearings on the rear. All checked by myself and mechanic. Any suggestion?
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In article
snipped-for-privacy@yahoo.com wrote:

Check for seized wheel cylinders.
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If you talk about brake cylinders (front & rear) they are fine.
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