bazer won't start

I have a 1997 Blazer 4.3L automatic that will not start. Went out the other day turned the key and NOTHING! Just a click and the fuel pump kicking in. The first thing I did was check the battery and conections. They are all fine from what I could tell. Next thing I did was buy a book on the truck and take out the starter. Took the starter in had it tested and was told the starter was fine. Took the starter home, put it back in and turned the key and it worked! Shut the truck off turned the key again and NOTHING. Where should I go from here. Even if I could afford a machanic I have no way to get it to them. Please help!

check wires, sounds liek a short, and try to post in the blazer forum.