1999 frd Expl. 6.0L, xlt, 4wd

Just developed a humming noise in the front end when driving. Seems to be coming from the front drive axle or wheels, not sure and not noticeable at low speed, under 20mph, but after that you can hear the humming get louder as you increase speed. Truck has 170,000 miles on it. Pulled the tires and inspected the front and rear brakes. Everything is tight. Have good brake pads all around, Greased the ball joints that were replaced, the one's I could, Checked the oil in the front differential, full, Checked the tie rods, and rotated the tires. Rotors were changed last year, and no vibration in the brake pedal when applied. I can see no leaks from the front differential and the CV boots are like they were when stock. No holes or tears. Still have that damn humming sound. Noticed that the emergency brake was applied and don't know how long it was driven like that, Not my primary vehicle, Released it. All work was done after releasing emergency brake. checked the back brakes, no harm. it's Kind of like, not a grinding but a race that's slowly dying. Haven't had a front end alignment since last year and never had the CV joints replaced, they are stock. I know this is long but I'm trying to think of all the inspections I've already done to give you a better perspective on any repair thoughts. I'm leaning on CV joints or front Diff. Do you think the grease in the boots deteriorated to a point where it's evaporated even though it's encased? Or am I on the wrong path in my thinking? Any help on this situation would be most appreciative. thank you for your time.