A similar thing happened to my son's Mustang just a few months ago. The crosspin broke, causing the spider gears to fall out while he was going 70 mph on the freeway, thus sending shrapnel throughout the diff. It was pretty ugly in there. Used axle was $350, labor and some parts were about the same. The shop I use for stuff I can't do myself assured me that installing a used axle was cheaper than fixing the old one.
P.S. After installing the axle, we found that 5th gear was non-existent. Apparently the rear end locked up for a second when it broke, thus taking out 5th gear. Used tranny, new clutch kit, flywheel, u-joints, labor, another $1000.