I've been tested for lupus and RA, and myasthenia gravis, and a whole bunch of other stuff when I was a kid. I had issues back then, that stumped everybody. Never saw a neurologist though.

Is "sensorimotor polyneuropathy" a diagnosis or a symptom of something bigger? Should I expect to do more digging?

And would MS show up on a lower spine MRI or just a brain scan/spinal cord mri? Does it show up weird in any blood tests? Would a CT scan show it?

I just realized that I was dx with gluten sensitivity 4 months ago. I don't know/think it's Celiac because the rest of my testing was negative except for the "anti-gliadin antibodies" test. Could it still be Celiac causing these issues? Do I need to opt for a biopsy or just go ahead and cut gluten 100%?

I'm feeling kind of cranky with all of this. I'm 23. This shouldn't be happening.

--is used as a diagnostic category by doctors for insurance purposes, but it is descriptive only; the term doesn't indicate cause, just symptom pattern.

You probably should expect to do more digging--maybe even into possible hereditary neuropathy syndromes, if you've had troubles for a long time, even back into childhood.

And yes, gluten sensitivity/celiac can be a cause of neuropathy. The Cornell Weill and Johns Hopkins medical centers, along with a number of European researchers, have been at the forefront of research into this. A positive anti-gliadin test does indicate you are making antibodies to gluten proteins. You should probably look at database known as The Gluten File:

https://sites.google.com/site/jccglutenfree/

It's unlikely a CT scan would show MS lesions of the brain and spinal cord; MRI is better for imaging soft tissues such as those. And sometimes clues to MS can be found in analysis of spinal fluid, or as the result of evoked potential testing. It wouldn't be a bad idea to have MRI's of brain and spinal cord done on sensitive MRI machines if you haven't already.

Yes, you should cut out gluten. Gliadin has the ability to penetrate your digestive tract, and spill toxins directly into your bloodstream. You name a condition, this makes it worse.
Read the latest research on this; unfortunately your doctors usually don't. Glenntaj's recommendations on testing the central nervous system are good; it it's all negative, the it's the gluten. The recommendation on the gluten file site is a good one. I recommend the book, Wheat Belly" by William Davis; it's written in plain English, but references the latest medical research.
Living a gluten-free lifestyle is not easy; I discovered wheat protein in my shampoo! But being vigilant and determined is far better than the pain and fear that accompany waiting around for doctors to decide what to do.