Rear Suspension Shock Absorber problems of the 2004 Ford Explorer Sport Trac

I am having a problem with the rear suspension on my '04 Ford Explorer Sport Trac. I have taken it to two different dealers in oklahoma city complaining of an uncontrollable bouncing when hitting certain bumps on the road at highway speeds and also speeds as low as 35 mph. This bouncing has occurred at least a dozen times and is gettting worse. When this occurs, it causes the rear of the vehicle to jump to the side about 6 inches, which has nearly caused me to wreck twice this month. Both service departments I've visited have described this as "normal" for this vehicle. I have ordered parts at my own expense to fix this problem, because I fear for the safety of my family. I am told by firestone and several internet sources, that my problem is related to weak shocks, which are specified by Ford for this vehicle.