I have the fix!! I just fixed this in my Cobra!!! I had a ton of play in the steering wheel, and it was ALL OVER THE PLACE WHEN IT HIT BUMPS!
There is a bearing in the firewall, that the steering shaft goes through. Mine broke, fell out, and was riding around the lowest part of the steering shaft. I could grab my steering shaft at the firewall and move it up and down. I looked at other Mustangs I have, and many had bad bearings in the firewall. Well, I finally found one that had a good one. It is held in a cup (with two 10 or 11mm bolts attaching it to the firewall.) A solid steering shaft will help as well, but I have had this problem in other 94-04 cars. Other cars, I just put a stock replacement shaft, it didn't solve the problem, and here I never knew of this bearing!! Well, I really looked into this on my Cobra, and I solved it!!
I have NO PLAY ANYMORE!! Car is like brand new!