Ok, I share your pain.
I had the same problem, it was a cracked weld on the passenger side cat. I saw several forums where they removed them and had them welded. No one I could find would touch it, also the guts were rattling around inside. So after wrapping the weld in high temp fiberglass tape to get me through till I could decide what to do I bought an aftermarket setup that dropped in perfectly! He's on Ebay, muffler express is his name (mufexpress)

Its a day job, $350.00 (parts) or so, be ready to replace the rear clamp where it goes into the muffler. I cut the old assembly out with an angle grinder. He guarantees the check engine light will stay off, mine has for 9 months and I never changed the 02 sensor. I got $85.00 back at the junk yard for the old cats.

Mine was NOT under the warranty, it stops at 10 years, 80K I have 120K, Benz was no help.

Good luck, its not as bad of a job as you would expect.

-John

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John
2001 ML 320
1988 560 SL
2009 Toyota Tundra TRD
1992 400 E (sold '07) (bought back, '13)
2008 ML 350