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I have a little bit of data that may help some.
Not a GM 10.5 but on a Ford/Sterling 10.5 under an Excursion (Super Duty platform) outfitted with 4.88 gears. My EX is our tow rig, we pull a '14 Jayco Eagle 338RETS travel trailer that is 41' long and weighs 11,000lbs for a truck/trailer combined weight of 19,000lbs. I monitor the diff oil (Mobil 1 LS75W-140 synth) with a GlowShift Engine Oil Temp stand alone combination digital/analog gauge with the temp sensor located in the fill port plug. We tow about 5,000 to 6,000 miles a year, all East of the Mississippi (so far) mostly in the warm/hot weather. Normal operating temps with this setup are from 150F to a high of 235F, the high was in 90F ambient pulling a 6%-ish grade for 2 or 3 miles at 65 to 55 MPH. After higher peaks like that it cools back down pretty quickly at highway speeds and even a light rain will drop the oil temp 20 to 40F. The only upgrade to aid in cooling was to swap on a stock '08+ Super Duty finned aluminum cover, nothing fancy and no additional capacity, just replaced the '05 original rusty sheet metal factory one. From what I could find back when setting the gauge up this gear lube is supposed to be completely stable up to something like 350F with a flash temp of 404F (207C). With my current setup and usage it gets just hot enough to cook off any moisture that may make it's way into the oil and the lube stays well below any problematic temps. I don't think you will have to have any type of external active or passive cooling setup, maybe just a finned alloy cover.
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