I recently had good success with wrapping the stock in plastic and pouring most of a bottle of mineral spirits in with it. I let it soak for ten minutes, agitating it occasionally, and then repeated the process.
Then I wiped it off, allowed it to dry for a day, and all the grease is gone, with no damage to the original shellac finish.
I tried it before with paint thinner and that did damage the finish (on another gun). Mineral spirits seems to be a different (better) product.