too many people to quote in a reply, but:
Yes, the center diff was locked.
The guy said they pulled the plugs and what they saw in the diffs was not good, so they're going by sight, not just by a guess or feel. They even said they were all shocked that both went at once. And I specifically asked if the transfer case was good, and he said there's nothing wrong with it. SO, I'm gonna call the shop tomorrow and ask a few more questions to see if they're BS-ing me, and I'm gonna talk to my friend's dad, a mechanic, to see what he thinks.
I found a guy on CL selling his 4.10 front diff and rear gears for $350 total, so I called him up and found that he's still got em. If the shop's diagnosis checks out, then I think this may be the cheapest route by far, which right now is what I need to do, and it should be the same as it was before. But does anyone know how much it may cost to get the rear gears installed/set up? I know that takes some expertise that I don't have.