I am not sure if the below sentence is actually passive voice.
Count I alleges negligence in Park Rangers’ removal of the signs.
If so - suggestions on rewording it without making the sentence very long?
No, this sentence is active. For a sentence to be passive, it has to contain a form of be and a past participle (like broken, worked, begun), but the main verb is simply alleges, an active form.
What you probably mean is that the subject of the sentence (Count I) is not an agent, but something that can't really "do" anything. But that is about meaning only: active versus passive is mainly about syntax, the structure of a sentence.
An example of a passive sentence:
In Count I, negligence in Park Rangers' removal of the signs is alleged.