I just bought a 2005 X-45 and had the dealer look into the hard steering issue when underway. During my test drive on the lake, the hard steering was the first thing I noticed about the boat. The previous owner agreed to have the MC dealer fix that at his cost. The mechanic who worked on the boat told me that everything looked good to him, he simply looked at the cable, greased it, and put it back in. When I picked up the boat, just like you mentioned, it steered fine on the trailer. In the lake, it steers fine at idle and while going at very slow speeds. As soon as you get underway, the steering is as hard as it was when I test drove it before buying it. I do not understand this either; it takes me two hands to turn the wheel with a lot of effort. My wife hasn't driven it yet but I suspect she will not be pleased with the way it steers underway.
I know very little to nothing about the rudder etc... I guess I am about to learn