I got my flowmaster exhaust a few days after I got my explorer when I was 16, that was 4 years ago and it still sounds about the same. Unfortunatley I didn't know there were different series of the muffler and sizes of the pipes. I just took mine to meinike and told them to put a flowmaster exhaust on. I only have a single out the side becasue they said my trailer package was in the way of doing duals (BS becasue I've seen plenty of duals on Explorers/Mounty's) So I'm not sure what series they used or the size of the pipe but it always sounded a little ricey to me. It sounds nice at startup and right on takeoff but then it's quiet untill pass 3000 rpm after that it has a growl to it that doesn't really sound like a flowmaster, not sure why but it sounds like a deep F1 car in a weird way if that makes sense. I had my sister floor it down our street and it has a wicked sound towards redline, but not that common flowmaster sound you normally hear which I think is weird, they might have made my pipe a little too big.
For your 91' I would go with either Flowmaster or Magnaflow (incredible sound on v8's but never heard them on a 6) I personnally don't like the sound of borlas (heard on a SOHC) but to each his own. The most popular choices are 40 series and 50 series flowmasters, the 40's being very deep and loud and the 50's being a little more mild. They also make a delta version thats about as loud outside as the regular muffler but with less interior resonance, I can't remember if they still make a delta for the 40's though. I would buy a muffler and have a custom shop make duals for you, just make sure to use the right size pipes to keep backpressure. Duals look a lot better on a truck than a single out personally. Definitley look into CGS, I did some research, and almost all of the showtrucks I found they all used CGS exhaust. It also talks about that on their website, dowload the video of the truck they have, I think it was a Dakota but damn it sounds mean as hell.
