@brayunique - This is not a backlight issue and as you’ve plugged in an external display we can see the issue is not present on it.
You clearly have a display panel level issue. On the backside of the panel is a small circuit board which paints the image based on what the GPU logic sends to it. This small board (T-CON) is part of the LCD panel as such you will need to replace the full assembly: 24” iMac (Alu) LCD panel,Apple P/N 661-4431 LG unit: LM240WU2 (SL)(B1) or (SL)(B2) or (SL)(B3) extension.