I had that pins and needles thing when I was a teenager, also on my upper back. But it went away. Needless to say I don't really know why.

You could always try all the weird allergies and intolerances to things that even overlap with foods considered safe in paleoland: eggs, histamines, salicylates, nightshades, etc. All of those can lead to the itchies. Although sensitivity to them is generally rare.

It could also be dry skin or something like that, but it just happens to act up once you put food in your belly. I have a recurring cough that always seems to be worse after I eat -- no matter what I eat. Good luck.

The (several) times I experienced pins 'n needles it wasn't digestive. Once was a pinched nerve in my neck, once was sciatica and once was an impingement in my shoulder.

I'm not saying your problem isn't related to your GI tract, but you might want to expand your possibilities to include a nerve/spinal cord issue.

I used to get that all of the time, all over my body. And it generally did go along with a lot of bloating/indigestion. Not exclusively, but it would often happen when I eat a lot of garlic. It happens very rarely now, but it did happen recently after a bowl of garlicky steamed mussels. I believe it is an allergy/sensitivity.