I agree that it sounds like arthritis to me although a geriatric panel is a good idea especially to monitor the changes in kidney levels.
Look for a good quality glucosamine/msm supplement. A lot of members have had great success with this product http://www.recoverysa.com/
. You use a double dose for a few weeks to begin and then go to a maintenance dose. You can order online or find it at most smaller dog food stores that carry the high end products. You can also ask your vet about cartrophen injections which I give my old girl and they help reduce arthritis inflamation.
Lastly, can I ask what you are feeding? A switch to a high protein, low carb diet is often very good for seniors. Supplements like salmon oil are also very helpful.