I was able to squeeze the diff in and out (completely off the vehicle, not just out of the way, which may not be neccesary for you - it will drop down and wiggle around once the mounting bolts are removed and you pop it off the side of the oil pan) by loosening the subframe but not removing it.

Remember that in addition to the motor mounts, the steering rack is attached to the subframe so you should remove those attachment bolts or be very carefully not to stress the steering hoses and steering coupler (both $$$ and PITA).

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David.X5
2001 X5 4.4i Sport
SOLD! at 160k miles