Humming/Vibration/Pinging noise when Avalanche in motion

[booster]

2004 Avalanche Z66. 108000 miles. I had to recently drive thru a lot of high water on a rescue mission to retrieve my daughter. Water depths approaching 4ft. This act has already forced me to replace both knock sensors. Since then, when driving at speeds above 60+mph I can hear a low frequency hum accompanied by a vibration I can feel in the steering wheel. The humming gets louder and the vibration stronger when I let up off the gas. I'm assuming it is the front wheel bearings. This 2nd issue I've notice occurred just after I drained and refilled the rear diff. I refilled with 80w-90 synthetic blend gear oil and the following day, after about 50 miles of driving, a pinging noise is coming from the rear of the vehicle during low speed acceleration. It almost sounds like ball bearings are bouncing around in the rear diff. I have no idea what is causing this. Any ideas?