You can indeed. Hyperpro do a progressive replacement spring. It will improve things to some degree as the stock spring goes limp quite rapidly.

However, the damping on the OE shock is quite frankly shocking! Just replacing the spring isn't going to fix the damping and, in some cases can cause a miss-match between spring and damping rates, making the ride worse.

If you can afford it, then a new shock, built for your weight/load requirements is the way to go.

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