dogcrazi

Hi, have a few questions, was hoping maybe to get some help. Why does it seem dogs are more aggressive when on leash than off? I had a wiemer, and on leash he was very aggressive towards other dogs. Off leash, he had no trouble with them.
Now i have a basenji mix rescue. I was told she was good with other dogs. When we got her home, i immediatly put her in the back yard to sniff around and get the scent of our unaltered male Lab, Mason. Salli is a spayed female Basenji mix. Both are 6 years old. We let our lab out to meet here. Both were off leashes. They bonded instantly. We have found out the hard way Salli is very dog aggressive. SHe wants to kill all other dogs she comes across. Why did she bond with our lab if she is so aggressive? What was so different about him that made her say "He's ok, but no one else is." Doesn't matter what breed, size or sex. She hates them all. I can no longer walk her without a muzzle. She attacked a extremly overweight chihuahua today. I just can't believe she is so agressive when she had no problems meeting our lab. Was it because they were off leash? It does not seem to be a territory issue. She attackes regardless of where we are. I don't know what to do with her. There is only 1 trainer in Lincoln NE where i live that takes on dog/dog agression, and I cannot afford her rates ($500) Anyone have any ideas?
Thank you all so much!