***Fixed. Lessons learned***
Got the cat in, seems to have fixed the problem
This took me about 3.5 hours to get everything removed, 1.5-2 hours to get it all back together.
I found having a stubby, flex head ratched, as well as a regular flex head ratchet, indespensible for getting into the tight space between the firewall/floor and cat flange. Also, WD40 and a 1/2" ratchet with extensio ns and a u-joint, for breaking the nuts loose, then switching over to to the smaller ratchets so you can get a more complete swing.
It would be a good idea to some M8-1.25 and M10-1.5 exhaust studs, flange bolts, and flange nuts on hand, in case you break one off.
In addition to the above listed ratches, 13 and 15mm regular and deep sockets, 13 and 15mm Gearwrenches/end wrenches, and a 3/8" Gear/endwrench is pretty much it.
The driveshaft does NOT need to come out, but there are 2 aluminum heat shields, one between the driveshaft and frame cross member, and one between the crossmember and the cat, that need to come out to give you room to work. This is what the 3/8" wrench is for. The shild above the shaft is plenty easy to get off, but the one immediately below the cat is a little trickier because one of the screws goes right into the crossmember right below the cat. Getting it out isn't too bad, but getting it back in without dropping it is tricky. Take your time, put the other screw for this shield in place first to help keep the trickier on in alignment.
The cat flanges that mate up to the exhaust pipe have studs that are press fit, like wheel studs. I checked five different places, including a dealership, no luck finding them. A place like Fastenal might have them, but they were closed. The M10 flanged bolts worked fine.
Since I had everything apart, I went ahead and changed all the exhaust gaskets:header-to-cat, cat-to-exhaust, and exhaust-to-exhaust. I saw evidence of exhaust escaping past the paper type exhaust-to-exhaust gasket, so decided to just replace them all while I had it apart. Seeing what the new ones looked like, there is no doubt all the taking apart and putting together done by the dealership had taken a toll on the old ones. Good idea to order these the same time you order the cat so you don't have to pay dealer prices.
The O2 sensor is easiest unplugged/plugged from the passenger side wheel well, unless you have a lot of one hand dexterity.
All in all, not a terribly difficult job, just tedious.