Someone welded the driveshaft to the coupler.
You are going to have to take the rear end out, or the engine, to fix it. It's going to be a pain. You'll need a new driveshaft. JeffinPa on this site is
the premier driveshaft guy!
Here's a link to his add. (PM him) Your also going to need a new hub for the motor end. He may be able to hook you up with that also. You may also find other broken pieces along the way. Let us know how it goes!