Ok truck sat up bout 8 months. Put on battery and oil change drove it home. Well after replacing fuel pump on the way. Now two days ago I go out get in to go to store and truck will not shift outta first gear. Changed fluid and filter no change. I'm new to Chevy brand and not real knowledgeable on transmissions. Trying to avoid big shop prices too. Any ideas?

 

Why was it sitting?

This sounds like you are saying that you went and bought a truck, then drove it some distance home. How much did you drive it before the trans won't shift out if 1st gear?

Could be something as mild as a rat-chewed wire that is now touching metal. The trans shift is controlled by the engine computer. I don't recall, there might be a fuse for the trans.

Could be as bad as a fried transmission.

Is the wiring to the transmission still connected properly?