Before you charge it, you will want to figure out why it's not blowing cold air. Is the compressor kicking on? Is the freon really low? Your system is sealed, and the freon should never be low if the system is working properly. If you are low on freon, you have a leak somewhere, and if you go and add more, it will just leak out. You will want to add a dye to the freon and figure out where it's leaking. Otherwise it will just leak right out again and you'll be back where you were before.

I forget where the lines are for your truck. The FSM just has some generic pictures. Hopefully someone else can chime in with specifics.