I had something similar; boat would steer OK at idle, but on plane not so much. took it to J&J Marine on 515 and they had me fixed and back out in less than an hour. replaced both seals on the hydraulic steering shaft, and used a machine to bleed the air out of the lines.
probably what happened to you; when you hit something, you damaged the seal on that side and got air in the lines. I'd replace both of them. it's a bit scary when you hit the gas and try to make a turn, and the boat keeps going straight!!
2014 Ranger RT-188
115 Mercury Pro-XS
Lowrance Gen-3 units