If vitamin d2 is made from mushrooms, can I get enough d2 by eating mushrooms? Do the mushrooms have to be specially treated first (exposed to sun?)?

Only Treated Ones. For mushrooms to be high in vitamin d, they must be treated with uv light. These treated white button mushrooms would be labeled as such in the market. Studies are still out on how much of the vitamin d is available to the body. Untreated mushrooms contain about 0.45 mcg vs. Treated containing 80 mcg per serving.
Mushrooms & Vit D. Foods are rich in d: fatty fish are mackerel, sardines and salmon; egg yolks; and d-fortified foods including milk and cereals. Mushrooms contain a compound called ergosterol that gets converted to vitamin d when exposed to uvb light. A new technique exposes mushrooms to high-intensity artificial uv rays for a few minutes. See: http://www.Care2.Com/greenliving/vitamin-d-fortified-mushrooms.Html.