it's no good "assuming" anything as this will only cost you time and money.
As other and myself suggest have a transmision diagnostic carried out before doing anything else, this hopefully will give you the faults code(s) to help diagnose the actual problem.
There are many possible causes,one the transmission fluid has already been mentioned, it is Toyota transmission specific and any other type will cause problems so make sure it is Type IV and the transmission fluid level is correct by following the checking proceedure in your handbook.
Then there may be a shift solenoid problem,again this should show on a diagnostic check.
There may also be a problem with the kickdown system this is normally a cable from the throttle control to the transmission, if not correctly adjusted problems occur.
So have the diagnostic check done 1st then come back with the results it is certainly a cheaper option than spending over a grand on an ECU which probab;y won't sort the issues.