Other than just replacing the muffler I believe your options will be limited. Not much place to run exhaust on the new Expedition underneath. I had a 2000 also and had more options on that model than I have had on my newer 2011.

I went with a gibson SS cat back but I do not think that will produce enough sound for you. I had the dual outlet gibson on my 2000 and the sound was perfect. Dual outlet will not fit on the newer models. Only single. I am happy with mine but could use a bit more sound.

Some of the lack of sound is due to the newer transmissions which shift at such low RPMs as well.