Synthetic...pfffttt...NOT!!!
I will never use full synthetic ever again.
I buy the cheap dino lube and change it once a year as part of my spring maintenance program.

Spike why no synthetic?

And thanks for the replies everyone, it's all been really helpful.

Should I be concerned with the brand? I can get 85w-140 in Coastal and Traveller, Castrol, and Lucas, they didn't have it in Rotella or Royal Purple. Also if I don't go with synthetic does that just mean I have to change it out more often? They don't have many synthetic options at the store.

I used the GM lube with the pleasant grape scent, lol I kid you not. Says it right on the bottle. Have a friend who works at GM so although its not the super gooey stuff, it is free. Just smells like nasty gear oil with a wiff of grape. Lol

Ok well I just got some Castrol 85w-140. Hopefully that will serve me well? How often will I need to change it?

Because when you go off roading/wading and you get water in one of your diffs and you have to drain out $130 of gear lube you feel pretty stupid.

How often should you change full syn gear lube?
I say every 50,000 miles, but I'm sure a lab test would say you could go longer.
As for dino lube, I say every 30,000 miles, again a lab test would say longer.

I change mine once a year, every spring after I go wading, its cheap insurance and it also keeps me intune with my truck.
Thats a good time for a full inspection under the truck.

I'd stick with Syn as that is what is called for. As mentioned if you are going to have the truck under water for extended periods of time and have the chance of getting some water in (would like to see exactly where it would enter for my own knowledge though) and have to change it out, I guess the regular stuff is OK. If you don't plan many river crossings though, I'd still say syn.

Ah I see. Well the thing is that there is no option to order the royal purple 85-140 synthetic online, at least at autozone. Plus none of the stores around my home have it. Am I in danger of anything with regular Castrol 85W-140? The t-case is not something I want to play games with. Just nervous about it i guess.

It does not matter if you use syn or dino, your transfer case does not know the difference. As long as the oil specs match what is required (GL-5 I think) then you are good. Don't over think it....

Now that is not fair! Over thinking things is *exactly* what this forum is about! :)