Yes, the nut at the top of the box can tighten worn bearings. Some of the nuts are large hex, and some have notches in which you can use a punch to tighten. Be very careful not to get the big nut too tight. I have never taken a box out to tighten this. The only ones I've ever tightened have had the spanner notches and I tightened them with the box in the car.

Good luck,