The third gear on this model (like the one i have) tends to hold for long when you aer climbing a hill. i noticed this. However, since you are saying that you noticed a behavioral change in the shift, it makes me worried a little... with my old pathfinder, when this happened, i ended up with a transmission failure after few months... but i can't recommend to change the trans fluid, since at 174000miles it might make things worse...

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BMW X5 3.0si 2008
Premium Package
Light Package
Navigation
Reverse Camera
Rear Sensor (retro-fit)
Comfort Access
Spare Wheel
Power folding mirrors
Voice Control w Bluetooth
Power trunklid

BMW X3 3.0i 2006
M Aerokit
M Sport Package
Navigation System
BT Handsfree
Xenon HID

BMW X5 3.0i 2001 --- SOLD