At 542,000 miles you are probably due for some front-end work; new ball joints, idler arm, pitman arm etc.
It's been done, much of that late last year and the entire power-steering/power brake system (complete with steering box) was replaced last Summer. On this van, the power brakes are hydraulic powered from the power-steering pump.
Last year about this time I had the ball joints looked at specifically because I was picking up savage vibrations from the front, found it was caused by (drum roll) a bad tire. Yeah, the problem I've been complaining about in this thread. Had the upper ball joints replaced, the lower joints were good. At the time I had the PS system replaced, also had the idler arms and pitman arm done. This thing was an expensive puppy last year.