The question you ask is really the root of neuropathy diagnositics!
Kind of like 'patents'...is it animal, vegetable, or mineral?
So many things overlap pure PN and other issues and vasculitis is definitely one of them.
That all said, I have to ask if you've taken a look at the Liza Jane worksheets and gone down the list of tests that you've had done? http://www.lizajane.org/
Also, I suspect that you are getting good care by your docs if they've gone so far as to do a biopsy....It was a punch biopsy yes? Not a full sural...I hope.
This below is a web site that can explain about the vascular and diabetic implacations and impact on the skin and nerves....please read it carefully - it just mite help you make the connections between causes and effects in your case: http://telemedicine.org/stamford.htm
Biopsies and nerve conduction studies don't always show 'signs' at first...I had excellent NCS's at first as I'd been fit before my own PN onset. Within a year I'd lost all reflex in my knees and ankles and the NCS's got worse progressively - to the point where during my last NCS I couldn't feel a thing! Many people hate these tests as they can hurt a lot - while one still has some feeling left. Me? I almost fell asleep on the table during the test!
I'm not up-to-date on inflammatory polyarthritis, but I'll try and look upstuff and see if there are connections... Have you had any spinal taps to see if you've other immune actions/reactions going on besides arthritis?
I hope this info will help you begin to sort out the neuropathy and other issues you have. This is a start? 's - j