I hae jacked up many of my vehicles via the differential, as it is the most robust central point in the rear of the car that can handle the weight of the rear chassis.
As far as safety is concerned, the footprint of the rear differential varies from one car to another, but its small area makes it likely to slip off the jack if you're not careful when raising the car.
As with any work being done on a car, use the jack to raise the car, then put jackstands underneath for extended work. A car on the jack on the differential alone is likely to teeter and topple off, espcially if you rock the vehicle while trying to dislodge a stubborn part...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car