I take a medication (Aromasin aka exemestane) that is well-known to cause osteoarthritis type pain. I also am a hospital nurse, working 12 hr night shifts, so on my feet constantly, and was a good 50 lbs overweight. Before paleo, I had a lot of pain/stiffness in my knees and ankles/feet -- my feet were ridiculous, I would literally HOBBLE when first getting up in the morning. I took Tylenol and Advil every single day, usually more than one dose of each.
I started eating fairly strict, lowish carb paleo (+ dairy + wine) in September and within a week noticed a reduction in my pain. A few weeks later I started CrossFit, and it seemed to improve further (although now I have new, exciting CF-related pain in my muscles!). I stopped needing pain meds at all for joint pain (do need them for muscle pain at times).
Since then, I've "slipped" a few times and eaten grains/sugars -- the biggest slip was Thanksgiving when the day before and the day of, I just said forget it (actually I said something else -- is swearing allowed on PH?) and ate crackers, rolls, stuffing, pie, drank beer, etc. (I blame my in-laws.) I woke up the day after Thanksgiving once again HOBBLING around like a 90 yr old -- the pain in my ankles and heels was excruciating. Immediately got back on the wagon, and within days, the pain receded again. Not vanished -- but receded.
It's a little hard for the scientific part of me to believe that 2 days of eating grains (I assume gluten is the culprit?) could do that much damage (or that a few days of refraining could cause that much improvement), but that's been my experience. I've also lost ~18 lbs, so that may contribute to the gradual improvement, but obviously not to the acute changes I had around turkey day.