While there is brass in the transfer case, it's a bushing that is not subject to the same film shearing issue of a synchro.

In my opinion and experience, GL5 will work in the transfer case without issue. Its not needed, but won't hurt.

I have never tried LSD additive in the transfer case, no clue if it would help at all.

87 Turbo Intercooled Raider, roller cam, torsen rear diff, LSD front diff, lockup auto with modified converter, V6 brakes, low transfer case gears...