Dave Ramsey says to sell the upside down car and get a loan for the difference owed. What if my credit is so shot I couldnt get a loan from a credit union etc...?

Who's Dave Ramsey? And why are you taking advice from him?

Keep the car, make regular payments on it, and it will help improve your credit score, as long as you are also paying all your other bills on time. And in a couple years, if you keep it up, you might just have raised your credit score enough so you can get a loan.

Well if you can't get a loan, then you can't get a loan - any difference owed (on what I have no idea though), would have to be paid in cash.

Then you pay the loan you already have. It is not that hard to figure out, seriously.

try a finance company

then you cant do it