The TD rear suspension is one that uses both the springs and the hyd unit to support the car. If one system is not working fully then the other has to do a lot more work... and wears out faster.. and springs only have a certain lifespan even just sitting still.. if the weight of the car is on them.... but in use you can figure 50,000-75,000 miles before the steel springs should be either replaced or rearched.... I know tons of people will howl at that.. but I am going from what car manufacturers used to list as the life of coil springs..
There are also stronger springs available from MB for rough roads. I have never seen a car which did not benifit from new springs...