Wound up with the same problem on my left cat. Looks like a manufacturing defect. Took the pipe off and had the welder at work run a new bead all the way around. Worked like a charm. He said it looked like the alignment of the weld in relation to where the casing and the end cap meet, was off center. The right cat was just fine. Considering I've got 130k on the odo and this is the first thing to go wrong, I'm OK with that

I did check Rockauto.com just in case and they have the left one (20400), complete with the one front and both rear cats for around $280 plus shipping and it's a direct replacement. They also have the right for about $100 less.