Here is a tail shaft out of a 727, there is a large Ball Bearing that supports the center of the shaft where the Key is in the picture.
904 uses the same type of bearing, we had a rough one on a 904 trans we were rebuilding that we had to replace. I might even have the part number yet if I can dig it up.
Notice that the Large Ball Bearing for the tail shaft is located just behind the speedo drive gear. The large bearing can be changed with the transmission still in the car.
If you want to try and see if the Large Tail Shaft bearing is bad you can pull out the speedometer drive, where the speedo cable screw onto. This will give you access to the tail shaft and the Large Bearing.
Look through the hole with a bright flash light to see if you can see rust or bearing damage.
Put your finger up in there on the tail shaft and slowly turn the driveshaft with your other hand in the proper forward rotation. If there is a bad bearing there you will feel the roughness in it as it conducts the vibrations to the tail shaft.
Side note: if you are going to remove the Speedo Gear to take a look at the Large Tail Shaft Bearing, take and mark the speedo gear aluminum flange (where the speedometer cable screws on) to the tail shaft housing with a dry marker. this way you can get it back in the correct spot to go with the plastic gear that is inside.
Prossess of elimination:
If you can determine that the tail shaft bearing is still good by this method but still feel a vibration or roughness of the tail shaft as it is turning then you know the damage if farther up the line into the transmission. And now it's time to pull the pan, see what is in the bottom, and check it out.
Any suspect transmission problems I have on any automatic car or truck, the first thing I do is drop the pan. This tells you the whole health of the transmission.
Pull the dipstick out of the automatic transmission and wipe it on to a clean white paper towel. Let us know if the transmission fluid color is
Bright Red or Brown ?