Ah, thank you for the quick and thorough response!
I forgot to mention in my first post I had already tried cleaning the laser with a q-tip, but no luck
As per your advice, I just installed the backup onto my 360 HDD. I couldn't get the disc to read, so I flipped my xbox 360 upside-down and I'm installing no problem now. (What the hell?!)
Anyway, I just booted up Brutal Legend with my 360 in its normal position, and it's playing beautifully now! (except for those nasty but unrelated in-game glitches
Thanks for the oddly simple yet effective solution to my problem!