Subframe bushings usually tell you when they need to be replaced. You should hear a clunking noise. Unless they are clunking it is hard to be sure they are bad without unbolting and lowering the subframe. As a rule of thumb they should be at least checked around 100,000 miles.

Did your mechanic give you a second opinion on the upper control arms? Based on the front suspension work you have done I wouldn't be surprised if some of the rear wear components, such as the upper control arms need to be addressed.

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X5 4.6 2002 Black Sap, Black interior. 2013 X5M Melbourne Red, Bamboo interior
Dallas