Does it not do anything or does it make a ticking sound? Also, do all the lights go dim when you try it? If it is ticking, then you have dead batteries. What happens is that the solenoid closes and reopens rapidly as the load from the starter draws the voltage down, causing the solenoid to open so the voltage goes back up and it continues in this loop. If the lights go dim, you at least have something going on.
I would agree on the starter contacts. If a hammer doesn't do the trick, some people have had luck with rotating the starter a bit and then trying again. This can be accomplished by putting the truck in a high gear and rolling it a bit, then trying to start it.
If you have access to a multimeter, that will tell you a lot. Obviously, the first thing to check would be battery voltage. Then, see whether you have voltage on the hot side of the solenoid. Then check the switched side of the solenoid by having a friend try to start it. If that doesn't do it, it is probably your starter contacts.
I hope that you get your truck running again soon.