The oil specified for the Spicer is actually Aviation grade mineral oil, more commonly known as "break-in oil" because it has no additives or detergents. I use it in my trans even though it leaks a bit because I happen to have some I bought for an airplane by mistake. You can find this oil at any small airport. You have to be careful with older trans because modern additives can destroy brass and other type metal bushings.
The rear uses GL-4 non-extreme pressure, most modern vehicles use XP so GL4 is not easy to find.
This link will answer your questions; http://www.mobiloil.com/USA-English/MotorOil/Car_Care/AskMobil/GL-5_and_GL-4_Gear_Oil.aspx