At the conceptual level, the vehicle design task is shaped into a concrete form. At this stage, not all vehicle parameters are typically known, so engineers must rely on experiential knowledge, recommendations, or already established design solutions. Within the scope of this research, a method has been formulated to for approximately determining the diameter of the half-shaft of the drive axle based on its damped forced vibrations. The diameter of the half-shaft was calculated using an optimization procedure, specifically by minimizing vibration amplitudes during the movement of an 18,000 kg truck over uneven terrain. Torsional stress checks were also carried out, based on which the safety factor was calculated. The analyses showed that this method is reliable and leads to solutions that fall within the limits typically observed in similar, already implemented vehicles.