I have one of the TRD fj's with about 22,000 miles on it. My transmission started slipping today after I changed the oil, I used full synthetic. Auto Zone put one of those code readers on it and it said there was no trouble codes. I am not sure how to check the transmission fluid cuz I can't find a dipstick for it and anyone else that I have had look can't either.Has anyone found one or is this a maintainance free transmission?

Should be maintenance free. At least 60,000 miles, unless you've driven through deep water or something. Watch out when changing your own oil - I understand that the transmission drain plug is easily mistaken for the engine oil drain plug. That would be bad.