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Also it's doesn't faster or slower with speed or engine rpm but instead with engine load if that makes sense. I think that rules out rotating assemblies. But maybe not.
If it's load dependent, then it probably is something getting moved by torque, and causing a bind or interference.
Have you tried different transfer case selections to see what the noise does?
Can you get it to occur by holding the brake and giving it some gas?
If you could get it on a frame lift, you might be able to get a better idea - especially if the brakes can duplicate the conditions for the noise.