Common problem. I've got the same mileage. Here are aome tips to follow in order:
1. Check fluid level. I had play, and then my pump started making noise. As I replenished, noise went, and steering tightened up a bit - air in the system does not improve handling!
2. The box can be adjusted. Careful - only very minor adjustment (run the "search" option on this board - this has been discussed at lenght!).
3. You could replace all the "wear out" parts attached down below - idler arm, the tie rod ends, and all that good stuff. I have not done it yet, but I hear it's all easy and DIY, and the parts will total you under $300 new. Some people say there is a dramatic improvement if you replace the steering damper - this is a part that is under $40!
4. No need to replace the steering box at this mileage whatsoever!
Search this board - there is a ton on the steering on the W126!
1989 560 SEL (black/black)
2001 Audi TT Roadster (silver/grey)