Have you noticed whether your chinchilla has also developed bald patches or if the skin looks inflamed, or is she just scratching and the fur is still thick?
Two weeks of constant scratching is definitely a reason to go to the vet - that shouldn't just be ignored. Mites are actually a common cause in chinchillas, especially if your pet keeps scratching the same spots like yours does (neck and ears). But fungal infections, dry skin from incorrect humidity, or even stress can trigger it too. The thing is: without a proper examination, you don't know what it is.
A real risk that's often underestimated - if you try any creams, oils, or "natural" remedies on your own, it can backfire with chinchillas. They have a very sensitive immune system and special fur that works completely differently than with other pets. Even harmless stuff can make infections worse or create new problems. Better to get to the vet quickly and then do what they say - whether that's treatment or just better humidity.