I don't know whether too much time has passed or not as I don't know the particulars of your former employer's LTD Plan. What you need to do is contact them, obtain an LTD claim form AND a copy of the LTD insurance certificate. Then review the language in the Plan certificate that defines disability for purposes of the Plan. The issue here is whether you met the Plan's definition of disability at the time you were terminated. You also need to determine how long one has to file an LTD claim following termination/being determined by your doctor as fully disabled. FYI, LTD Plans have a pretty stringent definition of "disabled" - much stricter than short-term disability plans.
You can't proceed with filing a claim without this information. You can't proceed with a suit for disability benefits against the employer's carrier until you have filed a claim and been denied.
A person, who is nice to you, but rude to a waiter, is not a nice person. (This is very important. Pay attention. It never fails.)