Mercon III and Mercon V are completely different. As for transmissions, you can use the newer fluid in the older trucks, but not the older fluid in the newer trucks. If the bottles state to not use in transfer cases, I would return them and get the Mercon III or equivalent. I have no idea why III ATF is okay and V ATF is not. I changed the fluid in mine with Valvoline MaxLife Mercon/Dexron ATF.
I went through the same thing a few weeks ago. Went to my dealers parts counter and asked for 2 qts. transfer case fluid, Parts guy comes back with Mercon V fluid. So i tell the parts guy that i need transfer case fluid not Mercon V fluid. The parts guy says Mercon V is the correct fluid. Just then the tech that did my studs last month walks up, So i ask him if the Mercon V is correct, He said I'll check he comes back with a notebook computer looks it up on fords network and guess what, The correct fluid is transfer case fluid. I'm Not sure whats the difference in the fluids.
The parts guy didn't know ford had labeled transfer case fluid.
He did look it up and it says Mercon witch my book says too. But Ford doesn't have Mercon label any more so he came back with Mercon V. The tech was able to access the part number for the fluid witch was labeled as transfer case fluid, Witch i believe is reg old Mercon.
The parts guy said he couldn't access the same info. the tech did.
I have a 2000 Excursion V10. I put Redline D4 ATF in my transfer case 7 years ago and change it every 50K miles. Before, when I put it in 4x4, the transfer case would make a slight woo'ing noise since it was new. The same with my old suburban before that. When I put the Redline D4, in just the first mile it was so quiet I couldn't tell if it was in 4x4. Even now with 205K miles it still runs just as quiet. Redline is a little expensive, but in my opinion $22 over the course of 2-3 years is chump change. I highly recommend it.