You can back off the lock. But I've not ever had to do this.
Best thing to do is to check that there is a 2mm gap between the ballstuds on the Ackerman rod. I use 4x0.5mm washers to do this, is stops the ball ends jumping on impacts too. Make sure you are using the Durango servo horn for the car, it's the length it is for a reason. If you are using the +5 castor blocks to get 30 degrees, you'll need about 3-4mm of washers under the ball stud on the knuckle. If the rest is built to manual, you should really be having no problems. It could be that your servo is over throwing. Just check you are getting full lock on the bump stop and you'll be fine.