Rear suspension sag

I own a 2000 Ford Taurus SEL, which I bought about 6 months ago. This car now has 100,000 miles on it. When I first purchased it, I noticed that the rear suspension sagged down just a bit, however I thought nothing of it at the time. Soon after, I took the car on its first road trip carrying 5 people including myself. Before we even took off, I noticed that the rear end of the car dropped unusually low, covering up inches of the rear tires. I had to get rid of a bunch of the weight in the trunk just to minimize the noise that was created from the car bottoming out. I have seen other late model Ford Taurus' with similar problems, where the rear end sags very low with just a couple of people in the back on several occasions. I have spoken with many people about this problem, with no resolve. Some have said that it is my springs that are bad, while others have said that my struts are bad, while some have even said that nothing is wrong at all. I strongly believe that something is wrong, and I need some advice on how to correct it. I could understand a little bit of a drop when weight it added, but my car tends to go much lower than normal. I am assuming that the springs and struts are all original.