I am upside down in a car; debt to GMAC far exceeds its value; do not want to harm my credit by turning the vehicle back

Legally, you have no recourse to reduce the loan--GMAC is not under any obligation to do so.

Practically, if you demonstrate to them that you cannot pay the current loan, they may choose to restructure or reduce it in some fashion, on the theory that it's better to give you a loan you can and will pay than have you walk away and leave them holding the car. You should approach them with proof of you income, assets, and car's value to show that you can't afford the payments as is and that it's in their interest to restructure.

If they won't restructure, declaring bankrupcy is one option--if you are just starting out and have no assets at stake, it would help wipe you slate clean and let you start over in a few years, once you've put the bankruptcy behind you. You also choose to default on the car, but note that GMAC would be allowed to sue you for the balance remaining on the loan (remaining balance over what they could get by taking the car back).

If you believe  dealer actually lied, tricked, misrepresented, etc. to you, you might have a fraud claim vs. the dealer.