I use Wal-mart synthetic ATF in the transfer case.It's cheap,works well,and I still change it every year.
Works great in most plow pumps too.
You don't need the limited slip additive for the front,only the rear,if your truck has a limited slip.
Change the oil first,and then drive the truck for a bit.If the truck works fine,your done.If you get any chatter,or wheel hop when turning,then you need the additive.Add a few ounces,and take the truck out and do some tight figure eights to work the additive into the clutch packs.Test drive it again for a day or two to see if your happy with it.If it still chatters too much,than just add an ounce at a time,and repeat the above procedure.
Your better off to have a little bit of chatter on hard turns when on dry pavement,as the limited slip will work much better.Too much additive,and it won't work at all.I find most good synthetic lubes already are slippery enough,or have the additive as part of the oil,so you will not have to add it at all.