Thanks to everyone who responded to my questions in my original thread :-) I think I have found the cause of the problem!
Three days ago I went through my calender and wracked my brain for everything that had changed in the household about the time Misty started scratching. I had changed the soap I wash dog dishes with, a strange fungus started growing on a windowsill and I had used poison on some ants near a doorway. Went through and cleaned everything up, scraped off the fungus, got out the old soap again (Dr Bonner's), washed off any traces of poison (not something the dogs really come in contact with anyway), etc. etc. Then when it was time to feed I remembered Misty's Proin. She was taking a small dose and I had upped the amount to one tab twice a day! Kicking myself for it now; it was still within the recommended dose, but I had never researched the possible side effects. Aggression is one and I think since Misty is very submissive this translated into a bad case of nerves and made her itch.
Needless to say she is now completely OFF of Proin. AND I stopped the Prednisone completely too. So far there is no sign of foot chewing, no scratching and her skin, while still bare on her toes, is a healthy light pink instead of an inflamed red. Hallelujah! I'm praying that it continues.
Misty seems calmer and happier so I don't want to go back to the Proin. My husband says what are we going to do instead, is there an alternative? so I guess I'll research that, but so far there haven't been any puddles to worry about.