There is no final commitment to lend until there is. Lenders have the money you want and generally write the rules you must follow to get it. Before I start looking for a co-signer however, I'd like to know what changed from the initial pre-approval to this conditional one? Have they changed your income, assets or credit scores since your initial application? If so why and are they correct? Sometimes a clerical error inputting something can create havoc in a file. Sometimes the way you calculated your income is different from the way they did. Once you know the reason they are asking you can either propose an alternative solution or be confident you did what you could.
Best of luck and if you don't mind, come back and let us know how things turned out.
This statement; is untrue:
" This should have come up long ago and the only acceptable reason it did not is because you did something in the past few weeks that either lowered your credit score or your income dropped etc."
It's not uncommon to get results of a 4506T back just before closing and there are things in those results that can dramatically change income. If this is the case, I hope you are doing an FHA mortgage or a Freddie Mac mortgage, because if it's Fannie Mae you have more problems than you think. Ask your loan officer the reason for the co-signer and how it affects your qualification.
If you're working with one of the big banks (BoA, Wells Fargo, Suntrust etc,) the chances of your successfully closing with them are 1 in 3 at best. Working with a big bank for a mortgage over the past 5 years is an exercise in futility, they cannot get it done
I empathize with your position and hope things work out for you.