Most of the time people usually only replace either the fuel pump or the level sender because they are 2 different parts but usually get connected to eachother. The reason people only buy what is needed is cause the unnessary part is more expensive when added to the cost. In this case, it appears places like autozone sell the whole assembly as 1 unit, the fuel pump and the level sensor. Together that would cost, $516.99. I found just the level sensor at a different website for $70. 25. I don't see any need to replace the fuel pump if it is just the level that is off.
Level sensor - Federal - Federal 08-09 $70.25
Here is another picture of it, number 26.
For, 2008 Chevrolet Malibu testing Fuel Level Sending Unit, click here.
For, 2008 Chevrolet Malibu Fuel Level Sending Unit, Removal & Installation, click here.
Also, if the code was P0461, click here for tsb 2008 Chevrolet Malibu Fuel gauge not reading full. It says, "The fuel gauge does not rise above the 3/4 mark after a fill up.This condition may be caused by a fuel vapor vent line (that is internal to the fuel tank)obstructing the travel of the fuel level sensor float arm."
Here is another tsb on, 2008 Chevrolet Malibu Fuel gauge incorrect, click here, it says, " If P0455, P0461 Set, Reposition Fuel Vapor Line.