Catalytic converter

It depends on what state you are in. In California and New York it is illegal to use a non-carb certified unit. Technically it is a violation of Federal law in the other states to install a Federal approved unit on a vehicle that was built under California emissions.
As such I try not to break the law and tell someone else to because the fines for a shop doing that work are very high.

As for the CEL, The design of the old system does not monitor the rear converter so the ECM would not even notice if it was gone from the system.

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Your Durango has two monitored converters. Those are the two that mount to the manifolds at each exhaust manifold. There are O2 sensors on top and bottom of both. Then you have the Y pipe and the rear converter. That one does not have any sensors on it.

The sticky part is that it is a california/new york certified truck. As such it's illegal in those 2 states to replace any of them with parts that are not CARB certified.

Early on this wasn't enforced very much but now there is actually a certificate that you keep and paperwork that has to be filed if you replace a CA/NY certified converter.
It basically says, "I know the law and have not replaced a CA/NY converter with anything but a CARB certified replacement, I know there is a big fine for violation of this law."

So say a person walks in and says "I'm never going to run this vehicle in either of those states". Maybe they won't, but what happens if they sell the truck and the new owner does?
Most shops will not break that law because of the fine. Even when you order the parts it will tell you if it's a legal unit or not.
(Not legal for sale or use in California; Not legal for sale or use in New York for vehicles with CA emissions)

Now another issue is fitment. Look at the images. Notice that your system uses 4 O2 sensors? Both up front? The Federal units don't have those, they use the single rear converter with an O2 sensor on each end of it.

You can't leave them out. You would also need to figure out how to stretch the wiring because the OEM wiring isn't long enough.
Overall you would be much better off to go online and order a system from a place like Rock Auto. You can get ones on there that will bolt on with no modifications for around $300.00. They have all three converters and bolt it on and call it done.