Will a fuel tank from a 04 Chevy trailblazer fit an 03 trailblazer

I'd say shop on line for both and see if the part is the same but there may be different sizes for each , start with the fuel pumps see if they match then take a close visual to see what the tanks are like . Still no guarantee .

Probably not. 04 and 05 Trailblazer are the same BUT ONLY if they're the same body style, have the same emission control systems, and the same options. A "Robust Fuel System" option, whatever that is, was being offered. The best thing to do is take the old tank to the junkyard and match it up. A friend got burned a few years ago with his 2000 Blazer gas tank. The junkyard sold him a tank out of a 2001 model and told him they were the same. Guess what, they're not! After doing some research I found that GM was using several different tanks during those years. So it's best to visually see if the two tanks are the same. Hope that helps! Jim