without being there to give it a test jiggle and feel..
I suspect that the diff spider gear is the one thats stuck the splines..
try this....
see if you can lock the drive gear or stick the chain to stop the diff from turning. see what happens when you try to turn the rear shaft. if its unexpectedly stiff then the diff may not be happy... in either case, if you can then stop the front sleeve from turning ( the bit that drives the VC) and try again if you get any binding thats the Center diff pretty much US....

