The 8.5 (an older generation of your 8.6) in my old diesel Blazer ran around 180-200F in 80-90F ambient, lightly loaded, with 4.10 gears and a Detroit Locker installed.
Smaller ring gears in axles with small oil capacities generate some pretty high temps with heavy loads. Also, lower gears run hotter than high gears. The rules of thumb I was given (and have largely observed) is around a 10-15 degree spread between 3.08 gears and 4.10s. This has to do with the taller gears having more tooth contact, spreading the load over a larger area, and they run a little cooler.
The faster you go the hotter the diff also.