When the catalysts were replaced, was either one broken up or missing a honeycomb element? If so, a piece might be left inside the muffler to clog up one side, and lower engine vacuum. Cut off the exhaust (or drill a 1/2 or larger hole) on the straight run prior to the muffler, but after the converter, and then look at your readings to see if they improve.
Just cut in a spot you know you can weld it back together.
Spoken from experience.
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