I was getting my car aligned recently and i noticed the rear thrust was out of specs by about 0.15 degrees.
I was wondering what exactly the rear thrust is, and if it being out of specs may be an issue.

 

one rear wheel is more forward than the other so the rear wants to go a different direction rather than follow directly behind the front wheels.

Do you have adjustable lower control arms?

 

if you dont have adjustable control arms it can be cause by a bent control arm or maybe even a bent panhard rod

if you do have adjustable control arms, adjust both of them till the wheel is centered in the wheel well on either side, that should be good enough