I had a small loan consolidated with my ex-husbands large student loan and he has deferred it to the point he can't any more and has not followed through on payment arrangements. He agreed when we divorced to pay it, I have that in writing but it...
Any time that you agree to be liable on a debt with another, you have probably agreed (absent some limitation in your loan document) to repay the entire. It's generally called "co-signing" or "co-borrowing" debt. So, from the creditor's perspective, you both are equally responsible for repayment of the entire debt.
But, as between you and your co-obligor (your ex-husband), you may be entitled to reimbursement for any payments that you make on the debt above and beyond your fair share. Here, with just two people liable on this debt, you'd argue that, at a minimum, he is liable to reimbursement to you for half of the unpaid debt that you paid. This legal concept is called "contribution."
The payment of the entire debt or liability is unnecessary, but the payment must have exceeded the share of the person seeking contribution. The amount you are entitled to may be increased, if he signed something in a divorce decree agreeing to a higher proportion of the debt.
Your remedy is to repay the debt, and then seek repayment of the amount you repaid in excess of your share from him. That's the difficult issue here: even though you have a claim against your ex-husband, you still are liable to the creditor for the debt, regardless of whether you ever get any money from the ex-husband.See question