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Hi James, I'm Clay a certified master automobile technician with 30 years of automotive repair and diagnostic experience.
From your description most likely the pinion bearing is worn and humming. If the hum and vibration goes away when the vehicle is coasting at 55 mph, pinion bearings are the most likely cause. Have the rear differential fluid checked and inspected for bearing and bearing race particles in the fluid.
I hope this information is helpful. If you have any questions or comments let me know.
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