I had a similar problem last year on my MTD-made tractor. Turns out there are 4 pins that attach the deck brackets to the mower. One of those pins had come out, causing the deck to hang uneven. However, you could not tell this with the deck all the way up, only when you put it down could you notice once side was dropping. After reattaching that pin and adjusting the leveling screws, all was well.
If you do have a bent shaft on yours, measuring the blade height to the floor, as DexterII described, will make it very apparent.
If you do have a bent shaft on yours, measuring the blade height to the floor, as DexterII described, will make it very apparent.