Not the back-light problem. Fault at its main digital board. Short circuited component/s at it. Contact any service technician; or the authorized service center. If you wish to get some details; check the site linked here. Surf the site with patience. Pull up older posts.
http://electronicshelponline.blogspot.com/
In most cases, the main board might have to be replaced. If so, a software [firmware] update too might be necessary.
Before going further; try a hard reset first. If it does not help you, replace the main digital board. There are two methods to reset TVs. The first is simple to do; a Hard Reset. The other is somewhat skilled; is a Factory Reset (System Reset). To make it a hard reset, just unplug the TV from AC mains wall socket, and re-plug it back after 15 minutes or so. If you do this by overnight is better. This procedure will make it a hard reset. If you want to make it a system reset [Factory Reset]; you have to enter its service mode option, and select the factory reset option. If you wish to get some details; visit the site linked here.

I didn't think the main board would be the problem since it passed the flashlight test, but it does make sense. I replaced the power board and the led driver board and even put the old power board back in and got the same results.

So it's not a power board or capacitor issue there. Also all capacitors look good.

I'll try replacing the main board (the only one I haven't replaced yet) and see if it works from there.

Thanks for the advice so far everyone! I'll let you know how it goes after the main board is replaced!