Keep in mind the wind and the road conditions

Test the vehicle, going in both directions, on a flat road.

Front end misaligned

Adjust to specifications. Refer to Wheel Alignment Specifications in Wheel Alignment.

Unbalanced steering gear valve (if this is the cause, steering effort will be very light in direction of lead and heavy in the opposite direction)

Adjust or replace the steering gear valve. Refer to Power Steering Pump Flow Control Valve Replacement - Off Vehicle in Power Steering Systems.

Steering shaft rubbing the inside diameter of the shaft tube

Align or replace the steering column. Refer to Steering Column Replacement in Steering Wheel and Column.

Tire Damage or Insufficient Tire Rotation

Replace the tire. Refer to Tire Mounting and Dismounting in Tires and Wheels or rotate the tires. Refer to Tire Rotation in Tires and Wheels.