I have a auto V6 with an open diff (no locker) and I am in the middle of this right now. My noise is constant, related to speed but with no change when I am on the gas, coasting or decelerating. I think my issue is pinion bearing but I will know more tomorrow as I am pulling the diff today. As you describe it, your noise really sounds like it is related to the mating of the pinion gear and the ring gear (either wear or if it has been worked on, backlash not set up correctly). I would first check the fluid level and see if it is low. Changing it is a good idea as it if will not only let you see the effect but will also allow you to see if there is metal in there. It will take ~2.5 quarts and cost you $20 if you use basic Valvoline gear oil. FYI...make sure you can get the fill plug out before you remove the drain plug.
Buckaroo