I noticed my old ranger used to do that and it was the tires in the back. One of them would always go down to about 25 psi. I'd only feel it vibrate w/extra weight.
If its your tires, it would sound a little bit like mild rumble strips w/weight on it. To simulate the load, you could lower each rear tire by about 5 psi and see if it makes the vibration. If so, then go 5 psi up when you have people w/you or for road travel.