Any type of helper spring will increase the ride stiffness under all conditions.

There are a least 3 types of helper springs I have seen in use.

One is a coil spring that fits over the shock absorber. The kit will include 2 pair of saddle type clamps that fit around the upper and lower portions of the body of the shock. Those springs require clearance around the full length of the shock body.

There are also kits that include some type of mounting plates and brackets to add a coil spring between the axle and the frame.

There are leaf spring helper kits that clamp to the existing leaf springs.

These kits could also increase empty ride height if they are installed with some pre-load.

Rusty