Does the noise change with gas inputs? Meaning, does it get louder or quieter as get on the gas, or off the gas? Usually, gear rearend noise will change as the input load changes, even at a constant speed.

If it gets louder as the speed increases, and thats the only thing that makes it more pronounced, I'd lean towards the tires.

However, I would find a nice dirt road,(if it doesn't bother you that is) or a road with a different surface, smoother or rougher, and see if the noise changes then. If its still there, and doesnt change, even on dirt, its probably not the tires.