As others have said, they are in equilibrium. However there is a physical/mechanical (mass transfer) barrier to this equilibrium. When a crystal is dissolving/being precipitated, all shrinkage/growth occurs at the surface of the crystal.
Once the equilibrium state is reached, the exchange occurs only at the surface of the crystal. There is no change for the material in the body of the crystal (except by diffusion, which in crystals is an extremely slow process.) For a radioactive tracer to reach the centre of the crystals, complete crystals would have to dissolve and new ones be precipitated, which is only likely to happen if there is a gradient of conditions (for example a temperature gradient) between one part of the vessel and another.
Also, if you have a layer of salt slurry with clear solution above it, the solid phase of the slurry part is only in equilibrium with the liquid phase in the slurry part. If you add a radioactive tracer to the clear solution, it will be quickly incorporated into the surface of the crystals in the top layer of the slurry, but take a long time to reach (by liquid diffusion) the bottom part of the slurry. And it may never make it into the middle of any of the crystals.
PS I am a chemist turned chemical engineer, and for quantitative design of dissolution/precipitation process, this distinction is important.