The original manual lists GL-4 or higher
for both the transfer case and the transmission. There was a TSB in 1993 that corrects that callout for the transmission to Gl-4 only.
Again, for the transfer case its a non issue. I have been running Gl-5 in my transfer case since 1994 without any problem. That includes inspecting parts when I installed lower ratio low range transfer case gears about 12 years ago.
I'm going to go out on a limb here and say that If I haven't had a problem in 22 years, its probably safe.
You do bring up a good point that GL-5 is a problem in some applications.
"Modern EP gear oils contain thermally stable additives that promote system cleanliness and do not corrode yellow metals under moderate operating conditions. The goal in formulating top-tier EP gear oils is to develop additive chemistries that will carry high loads under boundary conditions and protect mating surfaces from wear while minimizing corrosiveness to yellow metals and keeping steel gear components clean." Gear Oil EP additives
I have been running Redline 75-90 Gear GL-5 lube as well as Mobil 1. Same Redline in the front LSD and the rear Torsen. That simplifies what I need to keep on the shelf in the shop. GL-4 GL-5 TSB reference