I am having a very hard time finding a packet sniffer for OS 8/9. The only one I found was Etherpeek, and every download link I find for it (even on the main site) is no longer working. I can only seem to find the OS X version.
Have your tried Interarchy? I'm not sure if that is for what you're looking, but the pre-OS X versions of Interarchy are now freeware. It is otherwise quite a useful program for several things, so it is worth having. Go to www.interarchy.com.
There used to be an AppleTalk Monitor application. Would this be good enough for what you want to do ?
It was a System5 or System6 app that I got hold of some time in the eighties. I tried it out a while ago, it still worked under OS9. I'll try to find out more if it sounds like something useful.