Red Eared Slider, skin infection?

Question
A few days ago my husband pointed out that our turtles were getting scrapes on their necks from their shells. Um... I don't think that is likely. They are turtles, I would think they are built to avoid that kind of complication. So I looked at them and they all have kind of scabby patches on the backs of their necks. What is causing this? Much more importantly how can we heal them?
I know they are probably over crowded, we have 4 turtles just over a year old. I bought two and my husband decided we should have two more. I think he figured they would stay pretty small. So now I am trying to find good homes for two of them but right now I need to get them healthy. Any tips on healing them, and perhaps how to find good turtle homes, would be great.

Answer
Healing- boost water temps to about 80-85F, and try either 'Turtle Sulfa Dip' or a fish medicine called 'Stress Coat' per directions.

Also, review the basic cares to make sure you have the right water quality, depth, diet, etc. at a good site like http://www.redearslider.com

New homes- flyers in the ocal pet shops offering them for free to a good home seem to work the best.