My psoriasis is largely concentrated in my scalp, and it gets really itchy if I'm not diligent about washing it daily. Doing so is rough on my hair, but it's far better than the itchy, bloody, scaly mess that occurs if I don't. I know I also get psoriasis around my face (eyebrows, temples, ears), which isn't too comfortable either. I was doing some research on inverse psoriasis the other night, and I suspect that some nasty, persistent rashes I've had may have in fact been psoriasis as well. Lovely condition, isn't it?
As for scrotal tissue, years ago I had some bad itching and scaling down there, with very raw tissue beneath said scales. I didn't know what it was at the time, so I just kept the area clean and waited for it to go away. Wasn't fun, but that's what I did.
I'm still pre-everything, so I can't say much about psoriasis in one's vagina save for "eegh. That doesn't sound pleasant." You might want to talk to your dermatologist about that.