I think one should do what they feel comfortable with...but when BMW says that you can (see post #6 in this thread) as long as you lift from beneath the rear diff (making sure you're not putting weight on the diff's cover)...along with looking how the rear diff fits within the rear axle subframe assembly (see pics below). Plus once you lift the entire rear end...you can place jack stands below each rear (left & right) jack pads to rest the vehicle on while doing the work...which would relieve any stress on the rear diff/rear subframe assembly. If one uses common sense...lifting from beneath the diff seems doable. But again, only do what one feels comfortable in doing.
{e53 X5 subframe assembly with diff installed}
{e53 X5 subframe assembly with rear diff NOT installed}
