Answer: I wonder if there is an air pocket in the soil making the entire root mass sink in one direction? (This might account for the stress the leaves are showing.) If so you may be able to straighten the tree by saturating the soil and then stepping on the high side. With any luck you'll straighten out the rootmass and plug up any air pockets. You might also try probing the soil on the low side with a cedar or metal stake, as if you're tamping down the soil. If you find the soil has settled on top but not in the middle or bottom of the original planting hole, you can add more soil and work it under the rootmass with your stake. If it doesn't work, you're not out anything. It will probably adjust and grow more upwards as it matures (to the point where you won't notice).
On the other hand, if you'd like to make some minor corrections you can do so with stakes and ties. Place at least two but preferrably three stakes and tie loosely. Then tighten the one or two a little to begin pulling it back into place. Adjust the ties every month or so. You'll want to keep some tension on the tie(s), but no so much as to cut into the bark. The initial tying and adjusting will be somewhat easier if you flood the soil around the rootball the night before. You may be able to step on the high side and shift the entire rootmass toward the right direction. If so, great; if not, there's nothing lost.
Best wishes with your tree!
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