don't know if this would work, but you could dry dragging a toothpick with a darker brown food coloring through it to give it the wood grain look?
If you want it to just look like a 2x4 you could draw it on the cake....fill it in with frosting and press it down to kind of smooth it.....then paint it....if it's a new 2x4 use a very diluted yellow brown....maybe draw in a knot hole or two.....use a darker brown to shade....still very diluted....I just use vodka when I paint..but you can use lemon extract too. I just like a clear alcohol to paint with
Hummmmm, with fondant you'd have more options I think but with buttercream, well...that's a different story!! I like smashcakes suggestion of dragging a toothpick if you want a more "3-D look but I think I would try icing with a lighter brown BC first and letting it set up and "crust". Then I'd go back with an artist liner brush and dark brown food color to "paint" on the wood grain effect using a light touch so its staying on top of the icing. I did a full painting on a cake where I used that technique and it worked fine for me. (you can see that cake in my album titled Monet on cake-The Dance). There's no "woodgrain" on it but it shows how the food color went on for me with a brush. Are you making a wooden cross by any chance?
I hope I was of some help for you!!
blessings,
elly
Would one of the faux wood grain (from Plaid) tools work - maybe paint on some brown coloring and use the tool to make a design??
I used edible images on my humidor cake for the woodgrain.
Debbie
Ellyrae,
Your cakes are amazing! I just looked at your Monet cake and WOW! I am in awe. You are truly talented.
Thanks to all for your tips. I think I will try painting on the BC after it has crusted.
THanks,
dtmc
Thank you for your compliments and you're most welcome for the tips! I'd love to see what you end up doing for your cake!
Hope all goes well for you! 
elly
hi, this might be a lot of work, but it might turn out well, to make the grain in the wood, do a FBCT and line your paper using a woodgrain pattern roller tool from the hardware - "paint" the roller tool with the darker color to make the grain and then put your lighter color ontop of it - it might work, i know this tool works great to get a perfect wood look in chocolate. Be creative!
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