Heh. I have yet to see a working diff fall apart if you open it up. By the way, it's a 50+ lb centersection, he'd have a hard time *just* "opening it up" to see what was inside...
On some Chevy trucks with LSD, they come pre-loaded so tightly that the LSD doesn't slip hardly at all around corners. When the truck makes a turn, the shuddering noise would be one tire slipping as you move(or the LSD slipping internally), because the LSD is basically locking the rear diff up.
I'd say that if you only encounter this kind of noise while turning, it's the stock LSD being a real tightass, or someone welded the rear diff before you got it.