If it's coming from your gas tank I would, find a new or newer tank. That's just me. But ultimately if it wasn't leaking before the muffler replacement sounds like someone made a boo-boo. And didn't say anything?
Stay away from mud..........No but seriously I HATE mud
2000 TJ 1.25 RC BL, 3.25 RC Suspension, 33s for the shoes....