Don't be so sure about what the shop told you. I suggest you check at another shop. You didn't mention if they find an error code or not. Is there any seen damage, oil leakage in the tranny box?. What type of fluid you used when you flushed the tranny? did you top filled the tranny (topping till the fluid start to get out of the filling opening and the tranny temp is at near 50 degree). Recheck oil level. If there is a problem in the transmission you definitely will get an error code. see the code meaning. it might be related to a solenoid, valve body, incorrect shifting, torque convert fault etcccc. Don't go with the most expensive solution unless you are sure it is the right and only solution

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X5 3.0i SUV
2005 -113000 miles
GM-A5S 690R Tranny