Update...the Walker came Tuesday night and I put it in Wednesday night. The cats were definitely, beyond any doubt, the cause of the simultaneous detonation/misfiring. All of that is completely gone, the truck runs like new.
The Walker was a perfect bolt in fit with room to spare. No clearance issues at all. Wrinkle bends and spot welded heat shields look like [censored], but the truck runs really good. I'm happy.
Magnaflow did email me back and said there "may be a fitment issue on flex fuel vehicles." I replied confirming the truck was flex fuel, but haven't heard back. I don't think it's a flex fuel issue as much as it is an automatic transmission issue. A 3.0 is a 3.0 regardless of flex fuel capability or not. It's mounted in the truck the same and uses the same exhaust manifolds. I think though that all flex fuel trucks were automatics, and automatic 3.0s were by default flex fuel. I think the manual ones were not flex fuel capable. They probably test fit the cat I got on a manual truck(s) and it fit, but there is no possible way to get it into an automatic trans truck.
Either way, the Walker assembly took care of it.
For people with Fords from this era, it is completely possible to have a trashed cat and for the vehicle to NOT throw a catalyst efficiency code. My truck threw NO codes related to the cat, all it would do is flash the CEL. Wouldn't even spit out pending codes. The cats apparently were cleaning the exhaust that was getting through, they just weren't able to move the air through efficiently enough for the engine to run good at higher RPM. The OE cats didn't even look that bad on removal, but the truck runs far better now. It was an immediate difference...I hit the gas like I normally would to pull out on the road just after install and all it did was smoke the Wrangler Radials.
