Chevy AV with Loud hum in 4WD

I have the same problem on 2wheel drive mode on my 2003 Avalanche. I brought it to a mechanic for a different problem, engine shuts down while driving on a highway. After running diagnostics, he found that the fuel pump is bad. During removal of the fuel tank, he noticed the universal joint is worn and he also replaced it. He also checked the gearbox/ front differential and found that it was almost empty. I did not even tell him that I am experiencing some humming and vibration at a speed of 65 to 70 mph on the highway and when you let go of the accelerator it somewhat disappear. It's more pronounced between 65 to 70mph. Have your mechanic check the universal joint connections and oil level of front and rear differential. I feel very lucky to discover this honest mechanic who discovered my annoying problem even without asking for it. Try to get this done as soon as possible before it generates another costly repair. good luck..