As has been posted, 9004, for the non-sport trucks. Can't remember perfectly, but believe the sports used 9004 and 9007 ( 2 bulbs ). That would only apply to you if someone has converted it to sport headlights somewhere in the past.

Basically if there is only 1 bulb, it is the std. 9004, if it has 2 bulbs, it is 9004/9007.