scotlynkins, I would change out the rear diff fluid with a high quality synthetic (Amsoil, RP, RedLine etc) within the first year or 20k miles of service.
The manual for my '00 Silverado states (but dont quote me) for servere service and extended use of towing after vehicle purchase, to change out the rear diff fluid within 10k miles or 15k miles (something to that effect). Otherwise the diff fluid is supposed to be changed around 100k ( I will change it sooner).
Again, that's for my Silverado, though I have seen other manuals for trucks stating the same thing about changing out diff fluid quickly when towing right after purchase of the truck.
My guess for that reasoning is that the locking or limited diff, when under heavy load fresh from the factory, may have an accelerated break-in period, and clutch pack material or metal particulates may accumulate faster in the fluid.
When I changed the diff fluid in my '95 K1500, there was a magnet at the bottom of the diff housing covered with grey goop. Be sure to clean off your magnet too if so equiped.
IMO, if you are already using a quality syn (GC) I would do minimum 5k mile OCI's.
Like Jim 5 stated, I have seen posts here and other forums that the Nissan 5.4 prefers "thicker oil".
Valvoline Maxlife Synthetic is considered a "thicker oil" for a given weight. Even though its a "high milage" oil, its safe for use in new and rebuilt engines, per the label.
I use it wih great results.