Well, there are two things that spring to mind right away:
- General old age
Firstly, it's a sad fact that more "mature" dogs often suffer from unsteadiness in their back legs and lower back later in life.
That are a couple of breeds that this really affects more than others (Labs, Alsatians).
This is usually caused by joints drying up, muscle and tissue around the wasting away or thinning out. It could also be due to more serious conditions like spinal problems or even bone cancer.
But before we jump to conclusions, I'd recommend visit health store (yes a human one) and try to find a Glucosamine supplement that will help with joint problems.
The should work on dogs too, and the dosage is pretty much the same.
Pain relief life aspirin can also help with managing discomfort - and there are more potent options on the market too. Always check with a professional about doses and stuff like that though.
A visit to the vet might also be a good idea. Press them to perform an xray of your pup's spine and pelvis. This should reveal exactly what is going on.
As I said before though, this is a common problem in older dogs.. So treat your dog to lots of rest, relaxation, and cuddles.