Yea, don't use Mercon, -IF you can even find it. It's been discontinued since iduno, -2002/2003 ? It was the worst transmission fluid ever formulated IMO. It's a transmission killer. There's a long story (Ford history) to go with for those interested, but research would turn all that up quickly. Research that revolves around Synthetic developement.
Anyway, for the trans, transfer case and even power steering the best stuff to use is the 100% synthetic formulations. Merc V at the very least. If you want to step up and it's worth every penny IMO, - go with that Amsoil stuff if you can stomach the cost lol.
I did it back in 08, all at the same time. Yea, it was pricey, -back then I put my trust in another individual and it's paid off. Trans came back to life (it was almost shot, would barley move the truck) and it took all those old truck high speed vibrations away. A major difference in temps and that's what gives longevity... - as it prevents formulation breakdown.
So at least use a synthetic, - Merc V will work.