I had a squeak that sounds pretty similar to yours. It ended being a spot where the faux leather vinyl wrapping under the armrest was contacting the paint of the door's inner shell. There's a felt strip that lines the outline of most of the door on the inside to prevent that, but it had slipped out of place.
In any case, here's a video that does a pretty good job of showing how to remove the door panel.
https://www.youtube.com/watch?v=2z7p6FnQsX4
It leaves a couple things out, but you'll get the idea. The rear doors are essentially the same with a couple minor differences.