How do I change the transmission solenoid?wWill it give off a diagnostic code? on 1996 Buick Park Avenue

Car shifts ok,then when in 4 th gear,kind of shudders or shakes real bad, then goes on ok.usually between 50 and 60 mile per hour.Have checked for codes and there were none,but check engine light on.Seems like trying to pop out of gear,usually I ease off,slow down,car goes on ok.Car has 244,0000,miles.Just every once in a while,more often lately.

Asked by for the 1996 Buick Park Avenue
seek diagnostic at Aamco transmission and scan for codes.
Is transmission scan different than scan tool for engine?No money for aamco,going to be used replacement if necessary,high mileage car not worth spending lot....pretty sure its solenoid,found it for 21.00 thinking try to change it
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2 more answers
Next time it starts the shudder, lightly apply brake and see if it goes away, let off brake see if it starts again, this is a torque converter test! Post results for more help.
agree with ebans sounds like lock up solenoid is bad