It depends on the type of shocks you install and the weight of your truck. If you install shocks designed for a heavy-duty truck, they may stiffen the ride of a lighter truck. If you install shocks designed for a lighter truck, they may not be able to handle the weight of a heavier truck and may bottom out, which will also stiffen the ride.
If you are looking to soften the ride of your truck, you should choose shocks that are specifically designed for your make and model of truck and that are designed for a comfortable ride. You may also want to consider installing air shocks, which can be adjusted to provide a softer or stiffer ride.