Axles get completely replaced.
Here is what I found: It is certainly possible for any axle to wear out enough over time to need replacing. Since the suspension is built into a torsion axle if the suspension part was to wear out pretty much the only solution would be to replace the axle.
If the deflection, or amount of travel of the wheels, on the torsion axle has increased, caused the trailer to bottom out, or is not the same on each side then that would be a good indication that something is wrong.