Hi, Ginny111,
Hind leg stiffness is very common in older dogs of the larger breeds - labradors, GSDs etc., as a rersult of arthritis in the hips.
I personally find Glucosmine + Chondroitin to be useful in the longer term, but your vet may be able to help with an NSAID ( non-steroidal anti-inflammatory drug)
Acupuncture has also been shown to help in some cases.
Walter