Yes, the wheels will splay inward if the differential falls, due to the way the differential itself forms part of the rear suspension. That in itself is not serious.
What you have to do (if you want to attempt a fix yourself) is get the back of the car up on jackstands and find out what failed in the mounting. It probably came loose and then rattled against the mounting studs until they failed. You may be able to re-stud the differential and bolt it back up. If you do, check the mounting holes for wear - you may have to use (heavy) washers if the holes have deformed.
I used jackstands with wooden blocks (cut from a 2x4 and custom wood chiseled) under the rear subframe mounts. On a 300D that's near and just inboard of the rear jack points. You can also support the car from the rear frame rails (or whatever they really are called) near the rear sway bar mounts.
Disclaimer: This is dangerous work and you could be killed if the car falls off the jack stands. Have to mention that.