I have a Vizio D55-F2 with a darkened top half that appears exactly like the picture in the original posting. I disassembled the panel and replaced all defective LEDs, some of which fail open causing an entire strip to go out while others fail shorted allowing other LEDs in the strip to continue working. I tested all the strips before reassembling only to find the problem remained. I then began looking at the signals from the backlight PWM controller and saw that the darkened top strips were actually being PWM’d while the bottom half was full on. By luck, I disconnected the bottom strip’s 58V supply as a test and all other strips came on and, even at 50% duty cycle, it appeared to have a nearly uniform brightness. The darkened state also appears to stem from a possible power up test because the bottom LED row did not light when reconnecting the 58V supply while powered on and it appeared to be no PWM signal for that row as if it had been disabled. The 58V supply is from a very noisy switching circuit and I wonder if the fact that the mosfet for the bottom row is located right beside it could be affecting possibly the current sense going back to the backlight IC. It could also just be the backlight IC is bad, but it is a Novatek NT50511S for which I cannot find a data sheet. For now, I’ll live with it since the missing bottom row is hardly noticeable.