I've been working there for the last two years at nights while I finished up school. You probably won't be driving because the jobs are in fairly high demand; in Chicago here, you need about five years of seniority to be considered as a package car driver (the brown trucks). There's high demand for the jobs considering its harder as hell to find a full-time unionized job that can support a family.
I've loaded the entire time and quite like the challenge. It's not as mindless a job as I hoped ( I don't think any actually are) because there's so much red tape, shitty equipment and bull-shit safety rules but I've enjoyed myself. The pay is decent and the benefits are great for a part-time job, but just be prepared to work physically harder than you have in your life. I load about 1500 packages a night (~weight 35-40 pounds) and that doesn't even include all the 70-150 pound packages that get loaded seperately at the end of the night. The only reason I didn't quit earlier was some of my co-workers and supervisors were awesome and we turned the shitty brown mess into something fun.
My only advice for you is to keep your head up, work your ass off and don't take any shit from management. Go to your steward with any problems you have and they should be able to change things.