Some studies have linked EBV with Hashimoto's, but there's nothing conclusive. Some doctors have felt that Fibromyalgia is, basically, un or under-treated hypothyroidism, since Fibro and hypothyroidism share many symptoms and many/most of the symptoms go away if the patient is treated adequately with thyroid hormones.
From the reading I've done, there's no benefit of taking ranitidine for EBV when mononucleosis is active, therefore, I can see no benefit of it 15 yrs later, either. Furthermore, rantidine is used to block stomach acid and it's pretty well known that many people with hypothyroidism have too little acid, not too much, therefore, ranitidine can be contra-indicated for someone with Hashimotos'/hypothyroidism.
I, personally, took ranitidine for some time, when my insurance wouldn't pay for my prescribed acid reducer... I later learned that it wasn't an acid reducer that I needed anyway; I needed "more" acid, not less. This is a problem we find with many who have Hashimoto's/hypothyroidism. At any rate, ranitidine did nothing to help any of my symptoms.
If you have Hashimoto's, your thyroid will, eventually, be destroyed by the thyroid antibodies.
I believe information is minimal on the use of ranitidine, because it's not a proven treatment for any of the conditions you may have - EBV, Hashimoto's, hypothyroidism or Fibro.