The exhaust from the cat back is in bad shape but from the turbo to the cat is fine.

Is this due to a different material or something in the cat?

 

Don't know for sure but I'll bet it has to do with the heat generated between the turbo and cat keeps the moisture down so it doesn't corrode as easy. You probably get some condensation buildup downstream of the cat that causes corrosion. Especially if you are not driving the truck far enough or letting it run long enough to dry out the exhaust completely after startup.

Just my opinion.
Dave

 

I spoke to another guy and he said that he replaced from the cat back twice then went to stainless. From the turbo to the cat it does not aluminized, so not sure if it is different. It is common to just rust out behind the cat?

 

I just noticed the same thing. Mine rotted and broke after the resonator, and is in BAD shape between the muffler and resonator. Everything ahead of the muffler looks good though