Some have had a problem where the rear end of the front driveshaft (that connects the transfer case to the front differential) pops out of the transfer case due to a design with too little spline overlap. The "fix" is to get a slightly longer front driveshaft that has the splines fully overlapping.

I'm not sure of the exact symptoms of that problem, but if you search on here you should be able to find more about it and see if your symptoms could fit that problem.

Be careful when you're under the car when it's in gear, with an intermittent problem like this.

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2001 X5 3.0i, 203k miles, AT, owned since 2014