I was at 4Wheel Parts getting some stuff for my '99. I asked the sales rep what muffler setups sound good on the 3.4's. He just looked at me and shook his head as if to say "Dont even waste your time".
It's pretty tricky to get a 6 cylinder sounding right. I've heard some Ford 4.0 Ranger's sound ok. My 4.3 S-10 had a Cherry Bomb glass pack with no cat and it sounded good at low RPM's...but that's about it.
My neighbor has a newer Tacoma (I'm assuming 4.0L) and I'm pretty sure he has a 18" Magnaflow. It sounds terrible, and you know that goon has to get all over it on our 200 foot long dead end. Sounds like a coffee grinder thats spinning on a carnival ride all while Crocodile Dundee spins that noise maker on a shoe string.
...but let me know if you find something that sounds good!
EDIT: Found a video that perfectly displays the sound of my neighbors truck!
http://www.youtube.com/watch?v=3Kf_RYVt2XQ