A tear to an anterior or cranial cruciate ligament of the knee can cause this kind of acute problem as can invertebral disk disease. X-Rays and palpation of the knee under sedation can diagnosis a cruciate tear. X-Rays and possibly and MRI of the spine can help diagnosis disk problems.
A cruciate tear creates a very unstable knee which can account for the falling down and wobbling in a 96 pound dog, but, disk problems can cause wobbling and falling down if there is enough pain. I don't know what kind of medication that vet gave but the pain cycle must be controlled and this may require multiple medications, such as Tramadol, and Amantadine, as additions to the NSAID pain regimen.
If the problem turns out to be a complete tear of one or both cruciate ligaments, than it would be best to have them repaired surgically. The instability of the knee will only get worse, if unrepaired, affecting all the structures in the knee requiring stronger and stronger doses of pain medication.
He started Amantadine last Monday. Xrays show great hips and spine. Will have the vet check Cruciate ligaments tomorrow. last surgery for TPLO was March, 2007. This came on so suddenly. I found a tick this morning but he gets a Lyme shot every year. Thank you.