It sure sounds like the deck's bent. I've had that problem on a 72" Toro deck. I took out the spindle housing and pounded on it with a sledge hammer. That didn't work because there was too much spring in the deck. So I used a 7' gate post from a chain link fence which was schedule 40 and 3" in diameter. I pried the mounting area back in position best I could and shimmed the rest with washers. That was years ago and it still cuts fine today.
If yours is out less than 1/4" you cou probably get by with just shimming the spindle housing. Mine was so bad when I flipped the deck over you could see it was out of alignment.
To check alignment I laid a straightedge across the edges of the deck and measured down to the blades.