Thank you guys so much! I actually have called about 5 vets...the first one said they don't do payment plans, but refered me to the second, same thing....unfor tunately I can't prove stable income, my money comes from babysitting friends kids, which is not stable, and every penny goes to living expenses and feeding my pups. I'm about to start nursing school full time, so I can't get a steady job right now. But the vet school idea is great! I will be calling them first thing in the morning. Thanks!
Also, I think maybe I misunderstood the vet about actual knee replacements. I asked her how successful the surgery was with a big dog, and she said it didn't used to work that well, but now they put all new hardware in there and it's very successful. I guess I should have found out what exactly she meant.
I guess you are right about the aggression, he has just gotten much more possessive and growly (and snappy) with "his" stuff, i.e. food, toys, trash, whatever he decides. He and Ben, the one in the middle, have actually gotten in a few fights. I've read that sometimes this can happen at his age, 15 months, that he could be challenging for dominance, but the vet behavioral specialist said i needed to get his knees fixed before she could determine anything behavior-wise.
Again, thank you for your suggestions, I appreciate it so much.