It's been suggested that 75W110 Severe Gear is the minimum one should consider using in a diesel P/U's differentials. 75W90 and 80W90 were spec'ed decades ago for the common loads and power levels back then. The specs haven't changed since, but the loading sure has increased a lot.
I've been using the 75W90 and 80W90 in my diffs for 11 years & 260K miles. They haven't fallen apart, but they sure do get loud in sub-zero temps. Were I to start over again, I'd've used a heavier lube in the differentials.
I would suggest using 75W110 Severe Gear at a minimum. If you haul very heavy loads long enough for the diffs to get to full hot temp, even on an irregular basis, I would suggest using 75W140 Severe Gear. For the little extra you'll pay for the Severe Gear lube, you'll be getting a lot of extra wear protection. The pour point of both -110 and -140 Severe Gear is -48F; both will flow fairly well at the normal cold temps you see in WY.
Finally, my usual caveat. Only you can decide which lube to use. We dealers and lubrication experts can only try to give you the information you need to make the wisest decision.