Recently bought an '08 Titan S3 Kingcab long bed truck. Installed a used front diff. out of a 2012 Titan. Is this the upgraded unit or the same as the one removed? Also I understand the 4 catalytic converters often go bad and they are pricey. Is there any way to preserve their integrity? The truck has 110K miles and there's no flex fuel available in my area?

Dunno about the diff, but the rear O2's are the early warning system for dying cats, just make sure you don't ignore codes from them. Misfiring or chronic rich mixture are the only things that kill cats in a hurry, so attend to those immediately if they occur. If a cat disintegrates for mechanical reasons and not melting, you'll generally get an intermittent rattle that will turn out to be inside one of them.