Why is my screen illuminating only at certain angle openings

Unfortunately it sounds as if either the screen itself is going bad, the inverter is dying, or, and most likely, the screen cables are being crimped in the hinge and cutting off your video/backlight. There is no easy fix, and the solution usually involves replacing the whole screen assembly and hinge, so if your machine is covered by Applecare, I'd definitely take it in.

@rdklinc: I think your advice was best - the inverter cable was definitely crimped. I bought a new board+cable on ebay for super cheap, and tore down my entire MacBook. The old inverter cable was in bad shape (I even noticed the airport antennae insulation was cut through from the plastic of the hinge after all of these years).

I'm back in business - but to anyone else who attempts this, I used an emory board and filed away the small wire guides at the ends of the clutch cover (see steps 43 & 44 MacBook Core 2 Duo Inverter Replacement). I think they contribute to the gradual wear of the wires fed through the hinges.

Thanks everyone at ifixit, my laptop is good as new after 4 hard years.