For the sake of future dweb searchers, thought I would post the resolution.
Turned out my center muffler was also shot, so I decided to just have everything ripped out and start over again. If I had to guess from the looks of it, some previous owner had cut out at least one OEM cat and welded in a universal (which is the one that was rattling) and the cat-back system looks a lot like the one AB sells. Baffles in the center muffler were gone, so I was looking at a complete new system from the manifold back.
I found a place called Hi-Flow in Manassas, VA that was mentioned on a couple other forums. Nice guys, looks like they do good work. I feel like he gave me a honest deal on a new stainless Magnaflow set up. Out the door for $1200, which still hurts, but fair based on the other quotes I was getting.
For comparison, I also got a quote from my local shop (Independent Land Rover Specialists, in Bethesda) that wanted $1850-$2000 parts/labor to install the AB kit (including new O2 sensors). Meineke in Silver Spring wanted $2400 to do the job (not including new O2 sensors). Rayco in Silver Spring wanted $2000.
I still want to know what shops other dwebbers are going to that they are getting complete systems for $500. Or is that rounding down in the same way that I plan to tell my wife the job only cost $500?