transmission issues

hey all i got a 99 f150 5.4l i got a transmission leak on the tailshaft right where the drive shaft connects.here is my problem 1 mechanic told me that its the rear main seal and another told me its a bushing because i also got vibration but when i go online to look for parts i cant find any bushing or seal or tailshaft for that matter i hope that sumone can help im trying to find part name and maybe where i can get them from it would be very much appreciated