It's VERY common for the rubber ring to be taken off permanently. If that is the only trouble you have with your driveshaft, just take it off permanently.

It can be removed by disconnecting the front flex disk, disconnecting the carrier bearing and lowering the front of the driveshaft such that the front portion can be slid out. Pay attention to see that the front shaft slides back on in the SAME position. While the front section is off, you can slide the rubber balancer off of the shaft rearward.

While the driveshaft is apart, flex the u-joint. If it is not smooth on both axis, try putting a washer between one side of the carrier bearing to slightly change its position. If that doesn't remove the vibration, then box it up and send it to the driveshaft shop that Jim recommended.

Don't try a wrecking yard shaft because the manual transmission shaft is a different length.

Have a great day,