Perhaps you are confusing the Approved Gross Vehicle Weight (including cargo, which is ~1000 lbs heavier) with actual weight. The weight of the X5 is already right at 5000 lbs. This is from p 180, 2005 Owner's Manual.
Weights
Curb weight
Ibs./kg

X5 3.0i
with manual transmission 4,652/2,110
with automatic transmission 4,696/2,130
X5 4.4i 4,927/2,235
X5 4.8is 5,016/2275

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