I don’t actually have an explanation, but I can tell you what I remember about the OE bushings. The 4 lowers are the same and the 4 uppers are the same. The uppers have a thicker outer sleeve than the lowers and the steel liner of the uppers have a serrated end detail. I’m not certain why, but that is how I remember them.
The polyurethane bushings do not have a steel outer sleeve nor do they have a serrated end detail on the steel liner. I have 14,000 miles on the polyurethane bushings from
www.4x4parts.com and I am very happy with them. The ride is good and solid. If nothing else they will be a lot easier to replace than the OE bushings.