My 05 does the same thing, on highways and rough pavement usually over 50-55 mph.I also notice it driving slower making turns, the wheel starts cutting into the turn itself, not that I don't mind, its not like its out of control.
However I credit it mostly due to two different brands of tire, as well as the front pair being fairly worn. I'll also have the alignment checked when I bring it in for the 4WD servicing.