I have a single-in-single-out 40 series flowmaster on my V6 and I enjoy the sound of it. For me it was just 30 dollars for installation because my brother gave it to me but I'm sure it would not be to terribly expensive. As long as you leave everything else alone and just switch out the muffler.
lucky you. i guess i will just have to look around and see what i can get and for how much. nice lookin 4runner! i need to get some pics up as soon as i actually do somethin to mine. (i just bought it)