Shifting to unrelated field after postdoc? Neurobiology to linguistics
Disclaimer: I know nothing about research in neuroscience or linguistics, so my answer is about a general person who finished a PhD in topic X and wants to switch to topic Y.
I would go for options 1 or 2. Option 3 is generally not a great idea. A PhD means spending several years on starvation wages, and a good chunk of that would be spent rehashing things you already know, like research methodology, how to explore data and so on. A postdoc allows you to explore whether your true passion really is linguistics. Treat the postdoc as you would a PhD - you have two years to write a thesis (well, not really write a thesis, but get enough results to fill one). Postdocs are often given much more freedom to explore their interests so you could build your knowledge on the topic while not having to do like a decade of PhD studies.
I would suggest a fourth option: would you be able to get a faculty position in neuroscience and then start exploring your dream at your own pace? Faculty members are masters of their own fate (to an extent, certainly more than PhD and postdocs). You can decide that you want to change your focus, and as a faculty member you may actually find it much easier to do so.