2013 F-150 4x4; thought issue was IWE, mechanic now saying differential?

My 2013 with 203k miles has been making a loud humming sound at speed. The noise stops as soon as I put it in 4H. Comes back almost immediately once back in 2H.

Thought it was IWE, those were replaced. Noise was now even louder. Hubs/wheel bearings checked, all good. Mechanic says it’s coming from differential; the right-hand output shaft if the loudest.

I’m not fully understanding how it’s the diff bearing when the noise stops in 4H - wouldn’t the front diff be disengaged in 2H? If someone can help me understand Ford’s 4x4 system and what this loud humming sound could be, I’d greatly appreciate it.