Take a look here for possible fix or more info.

https://www.fjcruiserforums.com/forums/canadian-forum/435953-fj-automatic-stuck-park.html

 

First, and most importantly, you need a new (and competent) mechanic.

Did this "mechanic" provide any specific information on what is supposedly wrong with the transmission?

Second, have your new, competent mechanic verify that the transmission fluid level is correct. Low transmission fluid level will cause exactly the symptom you have described.

Is there any chance that you had your engine oil changed right before the transmission started acting erratically? There have been incidents where incompetent "mechanics" drained the transmission instead of the engine, and then grossly overfilled the engine when they added an extra 6 quarts of oil.

In any case, DO NOT try to drive the vehicle, as you will severely damage the transmission clutch packs if the transmission is extremely low on fluid.

 

Just out of curiosity have you been servicing your transmission, drain and fill, filter change at normal intervals, 50K-70K miles?

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