Sounds like the air cells are bad. The air cells act as suspension springs. When the air slowly leaks out of the cells, more hydraullic fliud occupies the cells, thus less air springing and harder ride. The cells last approx. 60K miles before slowly losing their effectiveness.
Your load levelling valve is probably ok as long as it is not leaking. Even if it leaks, it is no big deal unless it is pouring out of the seal.