1999 Yukon rear end hum

Hello all - New guy here... I have a '99 Yukon SLT and have heard a humming noise (no vibration) out of the rear end since I bought the truck three years ago. It usually happens between 60 and 75 MPH. My father in law has a GMC 1500 extended cab pick up w/full 8' bed and had the same hum; his problem was the rear end. I've had the U-joints replaced, the rear end checked, put on new tires, checked the suspension and even went as far as removing the roof rack thinking the crossbars were vibrating in the wind. I'm still hearing the hum - and over the time I've had the truck it doesn't seem like it's gotten worse. The noise has been fairly consistent over time. I really love this truck and I'd hate to get rid of it. Any ideas??

Many things can cause this, some differential noise, if listened to closely, have some normal low pitched whine. This can also be an issue with slightly mismatched ring and pinion setting, improper bearing preload or the beginning of a bearing failure. No way to truly advise/diagnose in this environment.