My Opinion.
Let me say that I am not a ASE Mechanic.
It's best to accept that I'm a highly trained unproffessional.
With this being said, please educate yourself before following any advice which I may post on this topic.
First off, dont waste time doing nothing when you have diagnosed an issue with an Xterras catalytic converters.
To be clear, ignoring a faulty catalytic converter has destroyed more than a few 4.0 Nissan engines.
To explain further, there is small valve overlap in our engines . Basically, there is a time when both valve are open and the vacuum created by the intake stroke can pull a small amount of the debris from the catalytic converter. It dont help that manufacturers are installing cats closer and closer to the exhaust manifold's. This debris will damage the cylinder wall and create an excessive oil consumption.
Ok, so what to do?
The cheapest option is to either remove your catalytic converters and knock out the catalyst from inside it, or cut out the catalytic converters and replace the pipes.
In either case, there are options for "fooling" your O2 sensors into thinking your cat is in place and working properly.
Example:
I'm told the removal of your cats can be illegal with fines up to $10,000.
I cannot prove this claim nor have I spoken to anyone which has been fined
(I'm from S-Ga and we dont have inspections either)
For what it's worth, you can replace a catalytic converter for apx $135 - $150 per each. (Rockauto.com)
To save some money, you can delete the rear cats and just replace the front and you will save some cash.