I've got a generic replacement muffler in there, done by some shop years back for around $60-$80 installed, and I know it's nothing fancy.

 

I had a universal Magnaflow put in mine when the stock cat started breaking up. It didn't say high flow anywhere on the box so I'm not sure if its high flow or not, but the exhaust shop only charged $70 to put it in. They just cut the stock cat out and welded the new one in its place, haven't had any problems.

 

Anyone have pictures of the undercarriage of their Jimmy? I'd like to see the cat locations and the end result.