Chevy Venture Van 2002 AWD

Hi- I have an AWD van- from the back end it sounds like either noisy tires on the road or like the muffler has a small hole in it. I have revved the engine and cannot hear any problem withthe muffler, now I am concerned in case its something more sinister. I had the AWD fluids changed in 2006, should they be changed again and can someone please tell me how to do this myself thanks.