Noise from 2000 Druango at freeway speeds..?

We have a 2000 Durango that makes a noise that seemed at first to be just tire noise from the tread hitting the road.
However, when we started listening more to it we noticed that it occurs most
at freeway speeds above 50mph and when the accelerator has some pressure on it.
Whenever I take my foot off the accelerator and 'coast' the noise goes away and comes back with even the slightest pressure on the accelerator.
Would this be related to the tires, engine, transmission, ball joints, misallignment, misbalanced tires, or a combination?
Any suggestions are appreciated!
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