This is a 4 year old border collie austrailian shepard mix - 4 year old female.
My dog is happy and in generally good health, but like she acts very much like a working dog and has lately developed a problem with her hind legs when you exerts herself fetching. If she fetches a tennis ball thrown at ground level she is fine. However when the ball is thrown high in the air or bounced high in the air, she jumps up for it, exerts her hind legs and soon thereafter one can observe her hind legs shaking slightly. It doesn't appear she is in pain, but I have stopped this activity until I get a better idea what it is.
Has anyone seen this before and is it considered a serious problem that can be handled w/over the counter medication and/or should I take her to the vet?
Richard
This is a 4 year old border collie austrailian shepard mix - 4 year old female. My dog is happy ... considered a serious problem that can be handled w/over the counter medication and/or should I take her to the vet?

Border Collies and Australian Shepherds are both highly pain tolerant. I would not expect yelping or even limping (much) until the condition is very serious. So the shaking you are seeing could very well be pain related. What you describe sounds suspiciously like spondylosis, spinal arthritus, and I would see the vet ASAP.
Diane Blackman