Diff is barely warmer than the tires. I wouldn't be concerned.
Maybe dumb idea, but, borrow a go pro, or an old cell phone, and bolt or ziptie it to various places under the car while you drive, and see if you can see what's vibrating by noise or visuals.
It's annoying that it only shows up when driving above 35mph.
Is it there when your car is up on blocks?
Chalk the tires, put the nose into a tree just in case, jack the thing up, and have someone else rev it up with the ass end lifted while you walk around listening for sounds. No point in theory-crafting when you can quickly identify what is actually making the noise, rather than relying on where it seems to be coming from, reflected off road surface and surroundings while sitting inside the driver's seat.