You could consolidation the loans with a federal Direct Consolidation Loan. The Department of Education will issue you a new loan and use the money to pay off your existing loans. If you include your defaulted loan, that loan will be paid off, and your new consolidation loan will be current. To be eligible, you must agree to either repay the consolidation loan with an income-driven repayment plan or to make three monthly payments on your defaulted loan before applying for consolidation.
While your credit limit might seem like the number not to exceed on your credit card, experts actually recommend that to minimize negative credit impact, you should only be using 30% of your credit allowance. That means if you have a $9,000 credit limit, you should not exceed spending more than $3,000 before making a payment. This might seem a little counterintuitive, but the reality is credit restrictions like this are put in place to protect you. By spending much lower than your credit limit, you decrease your interest payments and ultimately your debt.