I recommend contacting Sallie Mae and asking to whom you should address your written complaint/concerns. Therafter, send a letter to Sallie Mae along with a sworn affidavit, (have it notarized) stating that you did not sign the document. There is no need, at this time, to state who signed the loan document, only that you are not the person that signed the document. It would be helpful to have at least two other persons send in affidavits (again, notarized) stating that they are familiar with your signature, have examined the document in question, and in their respective opinions, the signature on the document is not in your handwriting.
In sum, it is better to take the offensive at this stage than to wait until it is too late to save your credit.