difference between upper and lower is the diameter and the length.
heres what i use as the lower bushing:
http://www.steelerubber.com/suspension- ... 70-3344-35the thing with the rockauto part is..from the factory those bushings are not supposed to be sleeved. mine came off w/o any metal-sleeve, both upper and lower are full rubber bushings. the only sleeved-type bushing (i believe they call it spring eye bushing) is on the front of the leaf spring. the factory bushings didn't came with the inner sleeve for the bolt as well.
so my guess is...this is another fail from rockauto (had the same issue with a waterpump the other day)...or maybe i'm totally wrong since i'm a total rookie when it comes down to suspension and maybe it doesn't matter if you install that type of bushing instead of the factory type??