There is a pretty simple blood test to absolutely rule out EPM. They can do a more complex spinal tap too but if she is to the point where she is symptomatic, I believe (not 100% sure) that it would show in the blood panel.
EPM could cause a 'shifting' lameness. In my old mare, it showed much worse on her hind but occasionally, she would show lameness in the front. Mostly because of always dragging her hind around.
A vet is going to give you much better options. I know they are expensive but in the long run, selling a lame horse isn't a great option either.