Ha ha ha. I had a Cherry Bomb Turbo on an old car more than a decade ago! They still sell 'em... and they're just the same. They definately make a 4 cyl sound throaty. I've seen them online for as low as $25.
If you want "loud", simply swapping out the muffler is going to be the first place to start... shy of just cutting stuff off.
I will tell you though, I've read many, many people on this board say that exhaust systems don't do much for the 6 cyl. I could go on and on about the intake, exhaust, and motor mods my straight 6 TJ has... but it doesn't sound agressive at all. It makes a little noise, but there's just something about this motor that doesn't "sound mean".
For a minimal investment, try it out. See if you like it. Keep your stock one hanging around, though. In my opinion, there's better things to spend your dollars on.
-X