It is easy to find something like a flexdisc torn up after experiencing vibration...and think that it was the cause of the problem.... when in fact it may, being a flexible, soft item, have been the victum of the actual problem.
If I had to replace one flex disc I would replace the other one at the same time. This is much like replacing one tie rod end... things like this are usually replaced " on principle" together...
This is aside from trying to find it something is out of balance or reinstalled out of phase at some time in the past....
Have you checked the part which goes into the back of the transmission to see if you have excess play there... or your center bearing ? ( all depending on whether your car has those items.. ).

By the way... there are different ' strengths' of rubber used on some flex discs than others.... and changes were made in which the factory wanted installed on some cars... long after they left the factory...
Check with Alldata.com for your car and read the TSB's to find out...