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There will always be some road crown drift unless you are on a perfectly flat road, some cars are more susceptible to this than others. Try putting the car on a flat road and see how it feels with no crown.
Tires can often cause a pull. Uneven tire pressures or the tires themselves could be the issue. Make sure the tire pressures are set correctly and cross rotate the front tires to see if that makes a difference.
Even though alignment angles may be in spec there can often be issues that still cause a pull. Rear thrust angle is an often overlooked angle, if your thrust angle is off to the left then the front of the car will want to pull to the right. Even if it's in the green on the printout or machine it can still be enough to cause a drift. You may try another GM dealer or alignment shop and see if they are able to make it any better.