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Is it possible that you are experiencing torque lock in the transmission?
On GM transmissions sometimes the park pawl will get stuck and not let the transmission shift easily out of part when there is rolling torque on the dirveshaft. In other words, if gravity is pulling the vehicle down (or up) hill when in park then it will put a bind on the transmission park pawl making it very tough to get out of park. The best way to test for (and prevent) this is to set the emergency/parking brake before you put the vehicle in park. This will put the torque of the weight of the vehicle on the brakes rather than the transmission pawl. Then shift the vehicle in gear before you release the parking brake. If it comes out of park easily then you know the problem is torque lock, which can't necessarily be fixed on a vehicle that large; only prevented by setting the brake.