Careful with the words.  Red line doesn't sell anything called "Super Shockproof" oil.  They have a line called "Shockproof" gear oil and in that line there are 3 different types.  "Superlight Shockproof" pours out of the bottle like ATF, "Lightweight Shockproof" pours about like 30-weight motor oil, and "Heavy Shockproof" pours like a 90 weight gear oil.  They all get their anti-wear characteristics from "...a suspension of solid microscopic particles as an extreme pressure agent..." (can you say sulphur?).

I put Superlight Shockproof in my transfer case after seeing the tiny passages in there that need to transport the oil, and knowing the very cold temperatures I see quite often.  If it weren't for the cold, I'd use the Lightweight Shockproof in the transfer case (I put Lightweight Shockproof in the diffs as there are no such tricky oil passages there).  I personally feel like the heavy shockproof would be overkill for any normal G application, but if you're trying to get a worn out diff to hold together a while longer, or want to do endurance desert racing...yeah, maybe Heavy Shockproof is your player.  There's no free lunch with viscosity.  Normally higher viscosity is associated with better lubrication in gear oils, but that goes out the window when you allow high pressure additives that the GL spcs don't.  If the additives are giving you the lubricity, then the higher viscosity is only adding drag and heat to the system.

All that said, I think the MT90 and MTL would both be just fine in the transfer case.  In my opinion, it's splitting hairs to choose between them, and either is so far superior to the dino juice that the box was designed for, you'd be fine with any of the above and never notice the difference.

Dutch, my memory isn't what it should be, but I think some of the reason for them steering you away from the Shockproof line is that it doesn't meet GL specifications for some things like corrosion inhibition.  I could be misremembering though.

-Dave G.