Can I retroactively seek LT disability insurance for a condition that began at a previous employer?

I worked for a company that offered LT disability insurance. I began having problems while employed there. I sought treatment but was given a rather passive diagnosis. The treatment continued to get a bit worse and I then left the company and it has still continued to get worse. It has now progressed to be pretty debilitating and had I been diagnosed accurately while employed with the company I never would have voluntarily left.