Amsoil has been making synthetic oil for many years and is very good but a bit pricey. A lot of people use the Mobil One synthetic gear oil because it is widely available and reasonably priced. If you plan on doing a lot of serious off-roading and severe duty driving, there might be cause to go with a high dollar synthetic gear oil like Amsoil and Royal Purple, but for the average Pathfinder owner, the off the shelf synthetic gear oils more widely available are more than suitable. There is not a whole lot of difference between 75W90 and 80W90. The "75W" and "80W" are winter ratings. The 74W90 will be slightly thinner at very cold temperatures but both with be similar at normal operating temperatures.
As far as the transfer case, Nissan calls for Type "D" ATF. Type "D" is the original Dexron formula and, along with Dexron II, is a slightly different viscosity than Dexron III. In a transfer case, I don't think there will any real performance difference between type "D" and Dexron III. In Nissan auto trannies that spec type "D," servicing with Dexron III has been known to cause sticking issues with the valves in the valve body. Valvoline MaxLife ATF is recommended for use in Type "D" and other Asian manufacturer transmissions that use Dexron II from the factory, as well as a number of other applications. Maxlife is synthetic based. For that reason, I'll be using Maxlife in my transfer case when I service it next week in my 06 LE. I already use Maxlife in the transfer case in an 08 SE and in both of the transmissions and have never had any problems.