I drive 75% freeway, 25% city, no towing or dessert(ie no extreme conditions) - it is likely that at 70k mi. the fluids for Power Steering, and front and rear differentials needed to be replaced? the service came to ~$400. Manual seems to suggest you do these only if driving in ext. conds.
70k - Fluids on Power steering and Fornt + Back Differentials on 2006 Toyota Highlander
If you plan on keeping the Highlander for a long time I would change the fluids. I do for my own cars and always have. I have never had my own transmissions and differential fail. Our long term customers at work also have little failure with regular fluid changes and good oils.
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Extreme conditions. What is that exactly? You should change your fluids at regular intervals becuase there is such thing as lifetime fluids. PS flush will certainly keep your steering rack in tiptop shape. Good luck