I know its late in the post but like I've said on other posts, UNLESS YOU KNOW WHAT YOU ARE DOING, DON'T ALIGN YOUR CAR YOUR SELF. Caster doesn't directly affect tire wear. It causes a condition called camber roll which is the change in camber when you turn your wheels. IMHO, extremely high caster causes that annoying outer tire wear in Ford Trucks with the Dual I-beam suspension. It also allows you to pull out of turns.
I like what theBlacksmith said about unequal tire pressures. Generally a difference of 2-5 psi (13.8-34.5 kPa) will cause a pull. If you've checked this, take is to an alignment shop. My bet is a broken swaybar or suspension component. ----
An optimist sees the cup half full
A pessimist sees it half empty
An engineer sees it twice as large as it needs to be.