Yes you can do it without pulling the trans but its a P.I.T.A. You have to pull the valve body and such out after you remove the pan and then unbolt the shift lever from the inside and pull it out. Mine had headers and I had my doubts but it will come out. Theres a seal in the case for the shift lever and a O ring on the kickdown shaft. The biggest pain is trying to get the notched bellcrank back in place without losing the spring loaded dog.
Its's no fun at all getting all the steel lines to the valvebody and servo etc in and out either. Then you have all the wondrfull trans fluid running up your arm but Im sure it needs done as they all leak there sooner or later.
It's not that technical but if your mechanical ability is limited or you have no patience then I suggest you find someone familiar with the FMX to do it for you. The FMX is laid out different than most trans in the valve body arrangement so I would stress if you decide to have someone else tackle it to make sure there familiar with them.
I'm sure this is not the answear you wanted to hear but I have done this job not so long ago and its still fresh in my nightmares, ugh I mean mind. mm
Ah... the countless times I've ripped my valvebody out of my FMX... hope I never go back there again... Oh, and working in the dirt is no fun... especially when the drop pan overflows fluid on your workspace...