2000 sable vibrating noise from rear when 3 passengers
 

I have driven with a full trunk and full backseat of luggage and never had this..The other night I had 3 people in the back seat and everytime I came to a light the rear would make this loud vibrating sound and I could feel it from the rear. Any idea what this is? Is it the rear shocks or struts? The rear sags a bit but doesn't drag near the ground as I see alotta old fords and mercuries.