Mine does exactly the same thing... It's a combination of bald tires, a sloppy idler arm, and catchy spots on the driver side front brake... See if lightly pressing the brake pedal (just enough to take any slack out of the calipers) at 50 mph calms it down any. Also climb under and grab ahold of the idler arm ((small steering link on pass. side, just under radiator)(While stopped, of course)) and see if you can wiggle it up and down. I've yet to see one that didn't need to be replaced.

90 MONTERO ... Beat, Banged, 4DR recovery and recon vehicle... just keeps going and going...