JRA + Celiac + Autoimmune Thyroid (Graves or Hashimoto's) is a classic triad, often seen together. Recurrent canker sores (that don't heal well, so they scar) are very common with Celiac.
JRA goes into remission during puberty in about 50% of cases. Mine did not, when it continues into adulthood they call it "chronic" JA.
I was diagnosed with JRA in infancy, Celiac Disease at age 35 and Hashimoto's at 38.
My advice would be to go back to the Mayo and get retested for Celiac Disease. If you can't manage the time/expense, call Mayo and ask the doc there to set you up with a gastro near you.
If you're afraid of the diet - it's not that bad - there's tons of gluten free food in the grocery stores, some restaurants even have gluten free menus. I just finished a big bowl of GF mac n' cheese, it was delicious!