Although I wish I could give you an example of this taking place in the US, from my research I am unaware of a large-scale Neo-Nazi rally occuring in the United States in the early 1990s.

While there was a small Neo-Nazi presence during that time in the United States, the majority of Neo-Nazi rallys in popular media took place in East Germany, shortly before the fall of the Soviet Union. The Monday Demonstrations, mass rallys of hundreds of thousands of East German citizens, were peaceful protests against the East German government. These rallys demanded many rights, although the focus was centered around freedom to democratically elect leaders, as well as freedom of travel within other European nations. These protests lasted for ~3 years, starting in 1989, and resulted in thousands of arrests and hundreds of separate, large-scale protests, of which a large number took place in Leipzig, Germany.

This brings us to the Neo-Nazi rallys in Leipzig. A New York Time article from the time states:

"Hundreds of skinheads goose-stepped through Leipzig today, shouting ''Sieg Heil!'' as they smashed windows and disrupted a regular weekly demonstration for German unity".

Although there were a number of Neo-Nazis present at the rally, only a small percentage of the 100,000 protesters actually supported the movement, with many chanting phrases such as "Nazis Out" and straight up destroying the flags held by their radical right-wing counterparts.

In the end, the Neo-Nazis had a small impact on the movement as a whole, which would inevitably be one of the primary contributors to the collapse of the German Democratic Republic. Many western news publications sensationalized this small minority of the population by posing it as a much larger portion of the movement than it actually was. This skewed view of a relatively small portion of the events occurring a the time is, to my best guess, what inspired the plotline of that Seinfeld episode.