Your description sounds like a wheel bearing going bad to me - especially since you have swapped tires and still have the noise. It could be a CV joint, I suppose, but the coming and going of the noise with changing load and the fact that you hear it at one range of speed, sure sounds familiar. My '92 has both bearings going bad. My left one starts to hum at about 33 mph, though, and it keeps making noise at all higher speeds. The right one just complains now and then. I have replacements sitting in the garage waiting for space to change them out. I have been driving mine for about 3,500 miles with the noise. And, they were noisy when I bought the car, so who knows how long they have been "singing?"
If Ol' timer is right, a trans bearing will be a whole lot more trouble, and checking for low fluid is surely a good idea.
I have had really poor luck identifying which bearing is making the noise. More than once I have been fooled into changing the wrong bearing. But it really is not such a bad idea to change them both anyway. One BC member said to jack up the suspect wheel, spin it and put your hand lightly on the spring to feel the roughness of a bearing going bad. Something to try.