After reading this thread, I've decided to offer my two-cents-worth.
Based on your photos and your comment that you couldn't move the vehicle in 4-hi or 4-lo, I think something catastrophic happened inside the transfer case. It may have locked up and, with the forward motion of the car at the time, that is, the rear wheels turning quickly, caused the drive shaft carnage.
If it were me, I'd drop the T-case and test it.
Your mechanic may have been correct in his initial assessment, but if you check around the **** yards, you could find the parts you need and save a ton of money. It really comes down to how much you like the car!