I thought i had a clogged catalytic converter once - it turned out to be a Turbo hose leak. It could be the turbo-resonator of you have a 2004-2006.
I also know someone who had a Dodge dealer (who used to service Sprinters) tell him that the Cat was clogged. $2000 later, the problem was still there (ie. not the Cat).
I am sure (if you have the ability to weld stainless steel) you could put a by-pass tube around the Cat/Muffler.