It looked to us (we didn't disassemble it because it couldn't be repaired at the track) like one of the CV joints gave up the ghost, rather than the axle itself.
However, this is the damage that necessitated a tow to the dealer;
Left is the rear of the car, right is forward. The differential casing itself was cracked.
The Captain said the dealer would have the car repaired by Wednesday - luckily the Captain and his lady planned to vacation in the Nashville/Memphis area this week anyways rather than heading directly home. So while costly, the situation is under control.
I suggested to Denis (the Captain) that he get in touch with Mike (GSO Challenger) regarding his axle assembly for sale. With his 6.1 and Kenne Bell, he's putting a lot of HP and torque on the ground (at least 200 rwhp over stock) and clearly needs to beef up the rear drivetrain.
BTW, at the banquet last night, Chuck (Bayou Tiger) donated his prize of the Hopnot kit and Speedlogix gift certificate to the Captain. That was very generous of Chuck, and demonstrates what a great community of car owners this is.