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It has actually been discussed several times on here. Just search door code and it should show you how to find it under the dash. I got lucky and the dealer told me mine when I bought it used.
There are a couple of good threads on here discussing it. I was never able to find it on mine, however. I ended up paying a locksmith $25 and he plugged a unit into the trucks and told me.
I printed out the "matrix" in this thread, and got my code in about 20 mins. There are also other threads that describe how to find the code on a box inside the dashboard.