I have a two fold problem/sound. Sound from left side of vehicle, but different sound depending on the speed.

There is going to be a lot of chasing in this diagnosis, but keep tabs and it will work out. It is possible that one the speakers has become partially lose form its' mount, then it resonates against the interior.
There are several modes for each pitch of resonant frequency.
Meaning the loudest will be slowest as it has the greater magnitude and they get higher in pitch as you speed up. So, there
are many suspect areas to look at.
Is the left front the only speaker in question? How many speakers does your truck have? Do you have an amplifier?

Inspect the speaker/s that you suspect and see if they are mounted tightly.
If this does not fix it, they are plenty of things to consider and then attack it by process of elimination. Body and suspension noises are hard to nail down.
If you had a warped rotator you would feel the pulsing in the brake pedal.
It could have a rock or maybe the wrong bolt was used to mount the caliper and it is rubbing the surface of the rotor because it is too long.

I will research/put some thought into it and get back with you tomorrow.

I've had the wagon for 2 and 1/2 years and the noise only started 2 weeks ago. The noise is there with the radio off. Is there a possibility it could be the differential since this is an all wheel drive car? At low speeds it almost sounds like a grinding gear. The steering wheel does have a little shimmy when you apply the brakes abruptly.

Your pads and rotors could be too thin and are vibrating against each other which is transfers to the car. They are usually semi-metallic. I has some pair of pads that were semi-metallic and they squeals like rock was in the calipers.
After checking for that, it was just the rotor groves as new rotors and pads fixed the problem.

See if the is hemi-spherical scoring or gouging on your rotors. If so, you will need to get new ones.