Here is what you do:
Find a good wood grain pattern or whatever on google.
Copy Paste that baby right into Illustrator.
Vectorize it (essentially Live Trace it, or go complicated and hand trace it yourself with pen tool). If you Live Trace, be sure to expand it after and delete any part you do not want.
Lock this layer and create new layer.
Hit your 'M' (shortcut for rectangle), make a rectangle.
On the toolbar that has all the arrows and pen tool and everything at the bottom you should have a few choices, so use this to fill your rectangle with a gradient.
Go to Window > Gradient, also pull up Window > Swatches.
In swatches tab make your color swatches that you want and make sure to select them as SPOT COLOR.
With your gradient rectangle selected go to your Gradient Window. Double click on the little circle of color, this will bring up a CMYK option for you, and the left side of this you will notice a grid of squares, select this and it will take you to your swatches tab, select previous said spot color.
Rinse and repeat this as many times as you want for whatever effect you want.
Once this is done, unlock your layers and be sure the wood grain is on top, go to Window > Pathfinder, and Click the Divide option. Note that if the wood grain is filled with black the wood part will come out with the gradient. If the wood grain does not have a fill, but has outlines, then the wood grain will get the gradient fill.
This process will only take minutes, easiest way I can think of doing it to get the gradient to go where and how you want it.
