94gt transmission vibration

[89TBirdSC]

Hey everyone. Just recently got a '94 gt stang, test drive went great so we got it for 4900. Great condition too. However just a week ago the transmission had started vibrating somewhat violently around 35-45 mph. I just today did a good tune up and oil change on the motor, since it looked horrible and there was little oil in the motor to begin with idiot previous owner! I have done some research in this, some sites say that this is common with the transmission, it will shake and vibrate, and that replacing the fluid and filter fixes the issue. I did go to Ford and buy the right fluid/filter/gasket, so I'm ready to go for the change, but if anyone else has had this sort of issue any input would be appreciated. Thanks!
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[Herbstang]

Just for grins and giggles have you check the mounts ?? could be a Torque Converter Clutch solenoid, or the shift solenoid is sticking. It might also be the transmission filter is partially clogged also check for a slight wobble in the drive shaft