After checking fuel pressure, compression, vacuum hoses and coils. Everything checking out excellent. I started thinking about how I could check the cats without a five gas analyzer. I came up with an idea.

I would first heat up the truck and check the temperature in front and behind each cat. I wrote down each temp then pulled a spark plug wire one side at a time. This lets a lot of unburnt fuel into the cats. I then measured the front and rear of each cat again.

Being there was a lot of unburnt fuel being passed through the cat they should get a lot hotter in the rear. While doing this all the cats reacted to fuel except one.

I then pulled that side (exhaust, flange and cats) off. While trying to get the nut off I noticed an oil leak from above leaking right into the flange. The stud was stripped causing the oil to leak into the exhaust pipe.

After getting the pipe out and cutting it open, sure enough the left rear cat was hollowed out.

Problem solved!

 

That was a sharp idea. Thanks for the update and info.