swerving front end

[king-of-battle@worldnet.att.net]

I've got a question (obviously) about my frontend.  When I'm driving at
freeway speeds the slightest move of the steering wheel will cause me to
swerve to that side.  It's rediculously sensitive.  It seemed that maybe
one of my wheels wasn't sitting straight up an down so I took it to get
aligned.  The only thing that was out of specs was the caster ( to be
technical- each wheel should've had 4 degrees and the left had 0 and the
right had 2).  The guy said it wasn't adjustable and that the car had
probably been hit.  But the chasis shows no sign of an accident nor the
body.  I'm sure it was more along the lines of a curb.  

What causes the caster to be out and is this my swerving problem???
I did notice that my ball joints are quite old with the rubber worn out
but I have no excessive play in the joints.  There's NO play in the
steering wheel and there's a new steering dampner.

HELP!!!!!