Work as many positions as you can in the building. For peak season 2010, become a driver helper. Apart from working a double everyday and therefore having a fatter paycheck all through out peak season, you will have the opportunity to make an impression on the management at the package centers where the drivers work. I've seen people become drivers because of a great performance as a helper.
Not that I'm saying that management pulls strings to get certain people hired as drivers...oh, no. Of course not. They would never. It's all about seniority.
In short: You don't need to work every position to become a UPS driver. But, if you want to make a career out of it, you want to work in as many positions as you can. Besides, unloading really sucks.