There is a certain amount that can be garnished in terms of income from disability/ssi but it is a question best addressed with an attorney. Any sort of legal help you will need has to be based on knowing what the specific financial information entails (amount owed, interest assessed, who is the carrier of the loan, etc) that you should not disclose in a public forum like this. You should contact the Oregon State Bar Lawyer Referral Service for a referral to an attorney in your local area who can sit down and discuss these matters with you. Although there is no low-income option available (through the Modest Means program), an attorney may be willing to work out some arrangement with you to help you on this matter.
Since you co-signed the loan, you bear a bit of responsibility so the sooner you can get in touch with an attorney, the better off you will be.
This answer is provided as a general opinion to a question posted on an internet forum. This does not create in either party the expectation that an attorney-client relationship has been entered into between the original poster and the Law Office of Reid Seino, LLC. Any information provided should not be solely taken as legal advice but in the context of general information. Please seek legal representation for any specific legal questions.