I'd look at the GS6X37BZ/DZ gearbox (BMW part# 23007563053) used in the late e46 iXs. The e46 shares the same 5-speed trans with the X5 and never received X-drive, so there's a high likelihood your old transfer case will bolt up. The e46 used the same transfer case its whole production run as far as I can tell.
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