Check the driver's side door jamb and you'll find a white tag about 3"x5". At the top center of the tag inside the door you'll find the month & year of manufacturer. On the bottom left corner, you'll find "MDH" in real small type, under the barcode. Next to that is a 6-digit sequence. It breaks down as:
My truck has 3-99 on the top of the tag, and MDH 030211. So, it was built in the 11th hour of March 2, 1999. I took delivery on March 19, which makes sense.
Incidentally, your paint code is down there next to the MDH as well.