Westie mix aggression toward American Shepherd mix problem?

For almost a year now, our 8 year old Westie mix, Willow, has been extremely aggressive toward our American Shepherd mix, Scout. We don't know why, because they were just fine before. Willow is the pack leader, and the other two dogs are submissive to her. It was a very sudden change. This is a huge problem for everyone! Scout is over 120 lbs, and Willow is around 20 lbs. If Willow sees Scout, she will run at her and begin what she plans to be a fight to the death.

So, we have a cumbersome, but necessary system of keeping the blood to a minimum. If Scout is outside, Willow is picked up and held while Scout comes in. I then carry Willow outside, her growling all the while because she can see Scout. The door is closed, and then I can put Willow down, waiting for her to get off the porch until I go back inside. This system is reversed if Willow is outside.

We have tried to find a better home for Scout since this has developed, but no luck! I know that spaying dogs can fix behavioral problems, but only in younger dogs; Willow is very set in her ways of hatred. And I know we have let it go on faaaarrr too long, but we have no apparent option but a dog behavioral trainer, and those are expensive!

Is there anything that can be done, or tried, or prayed for that can relieve us of this situation!?