Mike - I'm a little late but I'll reply anyway.
Each time you got the alignment done, did they have a way to check the rear toe? I know they're fixed/not adjustable, but that doesn't mean that they're pointing the correct way all the time. As the bushes etc wear down it moves the alignment. Obviously if it's a great deal out, you'd replace the bushes.
If they just looked at the front toe, then it could explain why it's still out. If the rear toe is out, you need to adjust the front toe to compensate for it.
We have a Hunter Hawkeye Elite machine at work (top spec fancy equipment ) and it measures rear toe, camber, front toe, camber, caster. If the rear toe is out but not adjustable, it then compensates for that when you adjust the rear so you can get the vehicle to drive straight.
But yeah, a sticky caliper is quite likely too!