I am also curious which way you would go with synthetic gear oil for our axles. I'm betting I should look into it soon as I'm pretty sure mine has never been changed...76k and counting!
For synthetic it's a tossup. For mixed running a 75w-90 works fine that a combination of running empty and occasional towing . For constant heavy towing I'd go with the 70w-110
I've been running the Amsoil severe gear 75w-90 in mine. I did notice on a recent really long trip while towing heavy out in the desert that the rear axle was warm when I stopped. Nowhere near as hot as it used to get even without a trailer on the dino oil that was in it when I got the thing, but it felt like I could go a tad heavier with the kind of weight I was pulling. I'm gong to put the 75w-110 in mine, but it's really a matter or overkill more than anything else
Either one is going to flow freely in your pumpkin below -50 deg f where dino gear oil has turned to molasses by the mid +teens and by around 0 it's not flowing much of anywhere