There are no Skilled or Unskilled jobs in the National Master, even though just about everyone thinks there is.
Your electronic time clock may list various jobs as 'skilled" or "unskilled" but that is just a UPS internal payroll thing.
"Preloader & Sorter" jobs pay a dollar more than "All Other" jobs. The extra dollar has little to do with the skill of the job. A part-timer who shifts package cars, trailers, TP-60s, drives a fork lift, scans or operates a computer is an "All Other" employee. (I'm not saying this makes any sense. Just telling you the way it is in the wacky world of Teamster-UPS part-time hell.)
A "Preferred Job" is any job you like enough to bid on. It has nothing to do with getting an extra dollar per hour. Loader and Unloader could be a preferred job to you, but it wouldn't pay the extra dollar.
Some buildings voluntarily pay the extra dollar to additional part-timers other than the Preloaders and Sorters, either because they are not nasty people, or because local market conditions require the higher pay rate to attract sufficient warm bodies.
Your regional Supplement or your Local Union may have an agreement to give the extra dollar to additional people as well.
See Article 22 of the National Master here:
http://tdu.org/node/1617