Not a lot of S670ds were sold. i have a welder that has welded some front cases for me & guarantees his work. Never had a return. Can't say if it is repairable till seeing it. Email me if interested
I think your problem is ring/pinion. I think if you had one wheel on the ground & one jacked up & both jacked up wheels turned without resistance that it wouldn't be in the final drives. I do have used FWD parts.
The MT21 ring gear was not used on any US models.It wasn't a common import. I doubt it is still available from Japan. Good luck. I do buy parts tractors.
There is a ball with a spring to keep the lever in the position you want it. It may be froze. If this is the problem, Tap the lever lightly either way to get it loose,