From another guy that I have gathered some valuable info from:
The shock lengths are different. 86/87 are same length at 16.25", 88/89 are the same length at 16". Longer dogbone(6 3/4") is for the 86/87 shock and swinger, shorter dogbone (5 7/8") is for the 88/89 shock and swinger. This is because of the shorter swinger on the 88/89. I apologize about the misinformation on the linkage pivot position, brain farted, was thinking of 450r stuff. When the swingarm was shortened on the 88/89, the dogbone was shortened to change the leverage ratio on the shorter swinger. That is also why the shorter dogbone is used.