My rear diff. calls for 80W90, but there isn't really going to be much a difference from the 75 and 80 back there... still, I'd probably try to go with what they recommend in everything.
As far as the transmission, you should try to use a GL IV grade oil in that, whereas everything else should be running a GL V(according to factory recommendations). You might want to order some transmission lubricant online from Redline. I am using their Redline MT-90 gear oil in my transmission and it works very well for helping with reducing shifting notchyness.
You should be able to find full synthetic 75W90 GL V gear oil at Pep Boys and Auto Zone from Mobil 1 for the front differential and transfer case. I had a hard time finding many oils on stores shelves in 80W90 for the rear differential, and had to go with Valvoline Sythetic blend oil in that since it was the only 80W90 I could find... but I'm probably going to be ordering some Amsoil online and putting it in there in the near future.
I use 75W-90 in my axles and Redline Mt-90 in the transmission. You'll be fine to use that oil in everything, but it might not work as well with the synchos in the transmission. So I'd just use it in the diffs, and stick with dino oil for the transmission or go with the MT-90.
My truck is an 88 4x4 with a 5 speed and I recently changed out all the oils in the driveline. I was running Mobil 1 75w-90 synthetic in everything and although it worked with that oil being a GL-5 and meeting factory spec, I was noticing a gear whine in 2nd and 3rd, along with a strange notchy feeling when shifting into 1st. Changed the tranny oil to Redline MT-90 which is a GL-4. Big difference. Shifts much smoother, no more gear whine, notchy feeling gone. While the factory specs say the tranny can use a GL-4 or GL-5, mine likes the GL-4 much better.