Many LTD insurance contracts require that recipients apply for Social Security disability benefits. Whether the LTD insurer is entitled to repayment depends on the terms of the insurance contract. Certainly you are entitled to an explanation of the demand for and amount of any repayment requested. You may want to run the specifics by an attorney who specializes in LTD insurance issues to make sure the insurer is entitled to what it is claiming.
Social Security benefits can't be attached by or assigned to a creditor, so Guardian can't directly take a portion of your Social Security benefits. 42 U.S.C. Sec. 407. Whether your long-term care insurance can charge you with an overpayment, however, would depend on the wording of your insurance contract. Even if Guardian can charge you, they can't take any of your Social Security money to make you pay them back. I recommend consulting with an attorney who specializes in long-term care insurance, bankruptcy law, or both. Good luck.