My van has 112K miles with steering problems

What is your question?

The steering is hard to control because the vehicle wanders from left to right. It's worse at higher speeds. I'm constantly having to correct it and it's much worse when passing a semi or in strong winds. What is causing this problem? Could it be low air pressure? Could it be damaged control arms and/or bushings? I've read about steering gearbox failure and that scares me! Any ideas? Thanks. Mo