Look at drug use.... antibiotics can cause neuropathy.
Flagyl (metronidazole) and fluoroquinilones like Levaquin and Cipro (sometimes used for upper respiratory infections).

427 is about the lowest normal with today's new range. The new testing ranges start at 400.
So your wife could help herself with some methylcobalamin 5mg a day on an empty stomach for 3 months, and see if she improves any. If she decides to take some B6, I'd recommend the P5P, which is the activated form. NOW products makes a good affordable one, and only one a day. P5P should be enteric coated to survive the stomach acid.

Some patients with neuro symptoms and MS-like diagnoses which are not firm, can have gluten intolerance. So going gluten free for a few months may show improvements. There is testing for this, and you can read more here:
http://neurotalk.psychcentral.com/thread1872.html

Some people get weird sensations following a viral infection, or vaccine. These may or may not go away, and some neuros tell patients that viral triggers may clear up on their own...but this can take a year or two.

PN is very complex, with over 100 triggers. So patience and a detective mindset, are necessary.