Your Ruby's story mirrors my Bella's story. She started limping in April at about 6 months old, after vigorous play with another dog. She has continued to limp on and off since then. She NEVER rests, as she's a wild one. The limp does not (I repeat,
does NOT) slow her down. The vet did xrays and determined that it's a soft tissue injury. Also, like you, they couldn't feel any problems when they manipulate her leg.
She's been on Rimadyl before, but the limp comes back, since she plays so hard. Anyway, I'm trying to keep her inactive for 6 weeks, and I'm giving her Dasuquin with MSM, which I think helps. We're in like week 2 of resting, and it's not easy. She's now running from me, which is extremely counter-productive.
Anyway, good luck with your sweetie! We can commiserate.