It turns out that the engine light is caused by an efficiency issue in the catalytic converter. I can get that fixed soon, after payday. (I do need to get it fixed soon, not only for the sake of the environment, but also because leaving my check engine light on means I wouldn't know if something *else* went wrong.)
But getting my tires rotated turned up that both my right side ball joint and my left side tie rod were dangerously close to failure. He showed me both parts, and there should NOT have been that much free movement in either. Now that they've been replaced, I realize just how sloppy my steering had gotten. Frog in hot water, methinks.
I think I'll be paying the extra to get the tires rotated and aligned at Firestone, appointment issues notwithstanding.