driveline vibration

How much is a substantial amount? Have you tested to see if the vibration is still present when the transmission is in neutral and you are moving? Does the frequency of the vibration change with speed?

There is going to be a bit of play in the output shaft as the condition of the rear trans bushing and the outer surface of the driveshaft yolk are the controlling factors here.

If these are the OEM parts, due to age, I would really recommend replacing the driveshaft yolk, rear trans bushing and the rear trans seal. Be sure to mark the rear pinion flange/driveshaft for original location.

If upon replacement of these items the vibration is lessened somewhat but not what you were expecting, flip the driveshaft 180* on the pinion flange and try again. There is a "sweet spot" there somewhere.

Good luck, GT

The mid RPM band of each gear? That makes me suspect either the engine or the input shaft of your transmission. What kind of transmission do you have?

If the vibration is due to the wheels, drive shaft, pinion angle, or output shaft of the transmission, then the vibration would be based upon the speed of the car and not the RPM of the engine. There will have a given speed where the vibration is most notable (usually); you're hitting the resonating frequency when this occurs.

Since the vibration seems to be dependent upon the speed of the engine, then I would suspect either the engine itself, or the input shaft of the transmission.

My advice is to do this. Drive the car up to whatever speed/RPM gives the most noticeable vibration, and then put the transmission in neutral and coast. If the vibration doesn't go away, then it is most likely a problem in your drive line (wheels, drive shaft, pinion angle, or output shaft of the transmission). If the vibration goes away, then I would suspect the engine or input of the transmission. If you have a manual, rev the engine with the clutch in and with the clutch out. See if one or the other produces the vibration.

another idea

You might want to replace your transmission mount (the rubber part at the back connecting to the cross member). I chased a vibriation for a while and it turned out to be the mount. Even looked decent, but was bad. Cheep piece to replace.