Essential rule of thumb is to work as directed. You can try and talk sense to the supe' about you having to do other people's jobs, but it almost certainly will fall on deaf ears.
It's best to just focus on your pull and, when you're done, if you're asked to help others then just shuffle on over and help finish. If you've still got work on the belt/line, though, make sure to point that out and ask who's going to cover you while you're helping whoever they ask you to help. Still go over and work as directed, just make it clear that they're going to move a problem from one pull to another if they send you to help without filling the gap left behind. And don't be a dick about it! Just keep your tone and approach casual, like you're all ready to help but you want to make sure your job's covered as well.
That kind of logic makes me think the drivers'd start helping with the sort when they show up instead of sitting around havin' pissing contests over who had the highest stop/piece count or miles the day before. I mean, we're all UPS employees here-- it's not like we have specific responsibilities or anything, we're simply indentured to the company and at the mercy of their every direction.
And then the feeders, man, how come they don't do anything after they drop off their loads? I mean, they're UPS employees so they should probably pitch in too, and don't even get me started on the clerks. When they come in all they do is sit down behind the customer counter, the lazy bastards.
I do agree that preloaders are preloaders, though. If you're directed to work in more than one spot, it's best to just suck it up and move on. The only thing that might help you talk your way out of it is high seniority and the increasingly rare reasonable supervisor that actually responds appropriately to a well-thought out argument.