It turns better in high gear because the mower is moving faster and wheels that are moving are easier to turn that wheels sitting still. If the shaft the steering wheel is hooked to does not turn when the steering wheel is being turned than the problem is in the wheel to shaft attachment. there is probably a disk in the middle of the steering wheel that can be popped out to reveal a screw. Make sure that screw is tight.
If the wheel and shaft move in synch there is a gear way down in the mower that is probably stripped out or on its way to being stripped. That would need to be replaced which can be expensive. Also make sure the rods that go to the suspension itsels where the wheels are at are attached good and not broken.
fix it dude, July 2007