Driveline vibration out of now where! Help!
It seems you have this "Huge Fear" of your transmission that you need to get over. If it needs to be fixed you take it out and get it rebuilt, the case is not going to be damaged. The overhaul kits are only 100 dollars online, plus any hard parts but you won't know this until it gets opened up.
Owning these cars you need a network of friends and associates that you stay in contact with so that when something happens you have them watching your back, and they can guide you along as to what needs to be done, and line you up locally to the people they trust to do the specialty work. Then do it.
So now go back and take the pan off and tell us what you find. If you don't I can't help you anymore.
This supposed to be fun, and part of the fun is being around all the good people that know how to keep the cars going. You can provide your experiences and support to them too when they need it.