I am so sorry to hear about your puppy, I hope my story will help you a bit.
We have gone through a similar issue with our two boxers, Tegan and Owen. Tegan is currently four years old and for the past 8 months we have been batteling a bad spine. The symptoms that you described are exactly what Tegan displayed one day after work. We came home and she was unable to move her back legs, she could not walk very well, she was in extreme pain when she tried to sit or lie down,...
We took her to the vet and she was immediatley put on pain killers and a muscle relaxing meds to ease her pain. We did x-rays and she had what looked like a slipped disk in her lower back. We left her on the meds for about a week hoping that everything would move back into place on it's own, but it didn't.
We took her back into the vet and we were left with two options, surgery (very expensive, and not 100% guranteed) or chiropractic work. We of course chose chirporactic work first. We found a human chiropractor that has been working on dogs for a while and was willing to take on Tegan's case (this was to be his most challenging case ever). After the fist exam with him is was determined that Tegan had two vertabrae that has actually fused together at the base of her spine that was put extra presure on the rest of her back, that's why she had slipped a disk in her back. The fusion had also pinched a nerve in her back that was causing pain. We also found out that the reason for Tegan's vertabrae to fuse was genetics (we later found out that Owen, Tgan's nephew, has the same problem.
To make a long story short, we started Tegan on the chiro sessions, we went twice a week for a month, and now 8 months later we are going once a month as a preventative measure. She is now able to run and play almost as hard as she did in the past!
I have also increased Tegan and Owen's diet with essential fatty acids that help their backs.
I truly beleive that the chiroprator saved Tegan from surgery.
Tegan 8/23/96-7/30/04 CH. CGC, THD, VADC, VAADC
Owen 5/9/99 ADC
Payton 7/18/04 AGN.J, AGN, SGDC, ADC, AADC