+1 on checking the Idler Arm Bushing.
Like Bill said, a little motion along the axis of the idler arm bushing bolt is fine- the thing to look out for is when the axis of the long bolt can shift from side to side. To check, get the two front wheels off the ground, and see if there's any play in the steering by pushing the passenger side wheel. if there is, look at the bottom of the idler arm bushing bolt. If it's wobbling from side to side as you push the wheel back and forth, you know the bottom part of the bushing is blown.
I recently had a problem almost exactly like tinypanzer's on my '84 300SD- whenever I hit a bump at highway speeds there was a chance that I would get a scary oscillation in the steering. Replacing my idler arm bushing cleared it right up.
As for the guide rods, I don't know anything about that... Johnhef replaced the guide rod ball joints where they connect to the subframe them on my car right before I bought it from him
