Not addressing whether it is a good idea, but you would have to change the gears in both diffs (front and rear) or else it will tear up something - probably the transfer case. The DSC would also not like that. People do change the ratios by swapping front and rear diffs with other models of X5, but they seem to go the other way.

Given the amount of work, why not get another used diff and the sell the diff you have with the wrong gear ratio?

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