I think the ujoint was for practical purposes just fundamentally semi seized. It normally on these cars only works slightly in service. So the driveshaft was continuously trying to centre whip or run off centres with the semi seized component. Easier situation for it rather than movement in the ujoint. The resultant out of balance situation created the issues.

Certainly one to really remember as there are a lot of driveshaft issues presented on site.

You can still purchase motorcycle chains both sealed and unsealed. The sealed ones seem to have a small o ring type device at each junction on both sides of the chain to keep dirt out.