There are a couple different cases depending on year. There is the New Process 231 (older), Borg Warner 1345, and the Borg Warner 1356 (in manual and electric shift versions). All will interchange from tranmission to transmission but the driveshafts may atach differently. The 1356 can have either a bolt on rear yoke or a slip yoke for example. This is determined by the specific aplication. If you want the strongest T-case you need to find a 1356 manually shifted with the bolt on rear yoke.
If you just wanted to know if you can take a T-case from a C6 and stick it on an E4OD, the answer is yes.
Ken