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The gas pressure is your shocks will affect the ride height of the vehicle. If the pressure is different between your old and new shocks, then your ride height is different and you suspension geometry at ride height will be slightly different.
I notice that my truck is sensitive to the crown in the road. On the right side of the crown, my wheel is straight up, on the left side of the crown, I'm 10-15 deg off center. Mine also has slight pull.
At the factory, they will do a 4-wheel alignment to compensate for any rear axle thrust angle. I don't know what the tolerance of that is for our trucks, but any misalignment between front and rear axles can cause a pull. If you've replaced your rear springs, you may want to get a 4-wheel alignment to see if you have anything going on at the back end, and correct for it at the front.