I agree the front splines can't fully slip until it is toast, but my experience, and others, report hearing "clunk" or other metallic sounds going from reverse to drive before that point. When I had that problem, and removed the front driveshaft, I found the teeth were about half sheared off, giving it a few degrees of wiggle when changing direction.

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