Not really going to comment on whether synthetic is better or worse, there are plenty of people who endorse both. From all my research and personal experience I feel the extra money is worth it. Did I notice any kind of MPG improvement or less noise from the diffs or xfer, not at all, but I still like using it, in the long run I feel it protects a bit better and will extend the life of my parts, perhaps I am just throwing money away, I really don't know.
To answer your second question 75W90 GL-5 will be fine in both diffs and xfer case. If you do go synthetic Mobil1 is readily available at most parts stores. Front diff takes 1.2 quarts, rear diff 2.9 quarts w/ center diff lock(4wd models) and xfer takes 1.3 quarts. That way you know what to buy, I just fill them until fluid starts coming out the fill hole then they are all set. Get a pump to put the fluid in, much easier than trying to fill from the bottles.