The cat converter checked out OK. In addition to the muffler, there was a small leak at the flex pipe below the engine, so had them replace that as well. That didn't fix my problem completely, as the tranny slipped going up a hill on the way home.
However, I changed the ATF fluid and filter the next day. Knock on wood, but I'm thinking that it may have done the trick! No slipping on its first test drive, including highway speeds and going up hills.
The test drive was actually to a shop to see read for any transmission codes. They found a code 53, which looks to be a knock sensor. Does anyone have more details on what may need to be done/checked with a code 53?