Vicky.......you will get no unkind words from me. I know first hand how difficult rehoming can be as I went through it with Lexie late last year. She suddenly decided she hated Halle to the point of drawing blood, & I too tried everything I could to change things...even contemplated keeping them separated 24/7 (taking turns with family time), but even that didn't seem right for us. Lexie is now with a family where I am confident and happy that she is very loved and living the life as top (and only) dog of the house. I'm sure she is much happier that way.
So do what you think is right or what you feel is necessary for everyone concerned. As difficult as rehoming is, it may be the right choice in the long run. The only idea I have for why this is happening and/or what to do would be that I've read that sudden aggression issues sometimes come up when a dog possibly has a hidden health issue. Have you thought about that? Maybe a trip to the vet for tests of some sort is needed to be sure there isn't a problem causing the aggression before deciding to rehome.
I know you must be so stressed and sad; good luck with your decision.
