I need some help on what this is.

I'm trying to figure out what this square thing is next to the gas tank, i recently replaced my tank an a week after doing so it would leak at idle after driving it for a lil bit, an it would leak when i shut the truck off for about 2-3 min. and its leaking out of the bottom of this thing. this is just a picture i found.

thank you.

That is an anti-roll valve. The way that this device is supposed to work is if your truck rolls over on it's side (or worse) it's supposed to move some sort of check ball into a position that keeps fuel from flowing to the carburetor. EFI models (and AT models) use a different style that does essentially the same thing but designed to handle the higher fuel pressure.

thank you