Toyota Repair: 1998 Toyota Sienna, express lube, toyota sienna

Question
I am at my wits end trying to figure out what is wrong with my Sienna Van. I have the entire staff at Firestone stumped. This is where my story begins. When I bought my minivan from Ebay we were told about a howl that the back brakes were causing. I took the car into Express Lube and told them about it. They gave me non ceramic brakes but did not fix the problem (even thought I told them I thought it was the drum). I came out with 3 noises instead of the only one I went in with. I took it elsewhere and they resurfaced the rotors (I think that is what they resurfaced) and still had the howl (again I told them about the drum issue with Sienna’s). My husband replaced the drums with new drums he bought off of Ebay and no more howl (YIPPEE). Now I have a vibration in the back of the van. I have had brand new tires put on it and new shocks and still have the problem. I told Firestone that the drum had been replaced and whether it could be a difference in weight or something like that. I was told that it would be a 1 in a million chance that it would be that. My question is does anyone have any suggestions as to what the rear violent shaking is at 45 mph plus is? My 3 little ones are getting tired of being vibrated. I just don't want to take it somewhere else to have them tell me it could be one thing and spending a bunch of money and not getting it fixed. I had the tires balanced today and still it is shaking. The guy at firestone looked at the front axle and it looked fine.  Any suggestions?

Answer
Before I get into everything that may possibly be wrong, do this one thing: have the tires rotated from the rear to the front and let me know it the problem is still the same or if there is a change from rear to front?
I am so curious about this that I'm going to give you my personal email address so you can contact me directly, I believe that if I have all the information I need that I can solve this problem for you.  
Contact: tedrttr@yahoo.com