Brownells sells a compound called "whiting" for this purpose. It is a white powder that you mix into a paste with a solvent like methanol. Paint it onto the stock. The solvent leaches the oil from the stock, and it is soaked up by the whiting. The whiting turns brown, and you brush it away. Repeat several times. This technique has the advantage of doing no damage to the underlying wood.
I have had pretty good success with a nylon brush and mineral spirits for most cosmoline on stocks. What I have worked on with this technique has not been soaked but rather coated, however. I am nervous about soaking a wood stock in a water bath or applying caustic soda (aka, "Easy Off") to wood. Just opinion.....
Clemson