Just out of curiosity, does it have power steering? You aren't trying to turn it with the engine off, are you? That would be almost as hard as trying to turn a car with power steering and the engine off, just a really hard workout.
Don't anybody thumb me down on this, the larger garden tractors from several manufacturers have had power steering, it's a valid question.
Look at the various pivot points and see whether any of them have rusted up. Rust expands as it forms and can freeze a joint pretty tightly. Just go around with some WD40 in a bunch of pivot points and give it a good soaking, work the steering back and forth - jack the front end up, take the load off to reduce resistance, and just give the wheel a good shake back and forth for a few minutes. See if things loosen up.
This next bit might be something to hold off on, if the machine is still under warranty: next you could take the steering train apart and clean and lube every joint. Check and replace bushings as necessary, apply fresh grease and reassemble. That can make a poor-steering tractor handle like a new one, but like I said it's probably not something to do with Sears service due next week. The WD40 ought to get you going. Don't tell the service guy you did that though, he might decide it's working well enough and just leave from there. You want him to dig in and see if everything's okay.
Steering is especially problematic with garden tractors. Part of the moving parts train is down there next to the mowing deck and the turning wheels, subject to lots of dirt and moisture, and part of it is buried behind the engine, up next to the cowl. Very little of it is designed for easy service and repair, and virtually none of it is really what you would call sturdy. Add into that the fact that you have to manipulate all of these moving parts with just your arms (instead of bigger and more powerful muscles like legs) and you can see how it can quickly become really difficult to operate if just a few things get out of whack. Making things even more complicated is the fact of the heavy engine and mowing deck right there near the front, ensuring that there's always plenty of load keeping those front tires firmly planted.