Try to find the threads here on transfer case actuator gear failures. It is pretty common for your year and gives similar symptoms. There is a no cost fix that involves flipping over a plastic gear to get it to an unworn region. Since you've got a few weeks, may be worth a try.

I have a 2001 that doesn't have the actuator on the transfer case so can't give any specific advice on how-to.

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