if you have a good driveline shop nearby that can remove the flange and flex disc from the Benz shaft and splice it to your Impala shaft.

I don't know if there is a problem with having a U joint at the trans with a flex disc at the diff???

I would try for a one-piece shaft, since the diff is rigid-mounted. You don't need the factory two-piece arrangment, and it's one less weak spot to worry about.