vibration while towing durango

I have a 2000 durango 4x4 with automatic transmission. recently I have noticed a loud vibration coming from the backend and it becomes very loud when towing my small boat. It also seems to get louder the longer I tow. but after I have unhitched and drive the vibration almost totaly goes away. I do not beleive that it is the transmission as the noise does not go away or change when the gears are changing. Would anyone like to talke an educated guess at what this might be?? Thanks