my name is Juan. For 3 years I have been coordinator of Autostop Argentina, and hitche some dozen of thousands of kilometers there. Gettinmg long rides is not only possible but the average case, since trucks travel normally big distances and some cities are located even thousands of kilometers from each other. It is perfectly safe to do it, just don't hitch from a poor slum out of a big city like Buenos Aires or Rosario. The rest of the country is perfectly safe, just common sense you know.
A girl hitching alone is no problem also. The same as hitching alone in Europe or elsewhere. Truck drivers will pick you up very frequently, and will NEVER ask for money. Waiting times, should be said, are longer than in Europe for sure, and middle class people is a bt afraid of giving lifts due to the crime they see on TV, but don't worry, we travel our country widely and there is no place that cant be reched by hitch hiking.