If you want to rule out the steering shaft it's pretty easy to do. Park the car, put the key in the ignition so you can turn the steering wheel without it locking. Pop the hood/get under the car with a flashlight, and have a friend slowly turn the steering wheel back and forth. Use your flashlight and watch the steering shaft. Start where it comes out of the firewall, and watch it all the way down to the steering rack. It should turn with the steering wheel. If the steering wheel turns a little, but the steering shaft doesn't, the issue is NOT your steering shaft and no parts past the steering wheel will fix your issue.
Focus on the area around the rag joints of the steering shaft (the rubber sections). Often the rubber breaks down, and one side of the shaft turns, but the other side doesn't turn as much due to the slop in the rag joint. If this is the case, the MM steering shaft will help tremendously (as will a new OEM shaft). If the steering shaft is solid all the way through, then move to the rack then out to the wheels. Look for things that *don't* move when the steering wheel is moved - these will be your problem areas.
Once you've identified the cause(s) of the issue, *then* throw parts at it. You shouldn't just throw parts at it until you know what is causing the issue. There is nothing more frustrating then dropping a wad of cash on new parts, just to have the same problem again after the install.