Can I be sued for a 2nd mortgage amount in Texas after a foreclosure?

My ex wife and I purchased a home in 2008 in New Mexico. We were divorced in 2010. I gave her the home in the divorce but both names were still on the mortgage.

BOA was the lender and there were two mortgages with them. The second of which they transferred to be administrated by Green Tree while I was still in the home.

My exwife got remarried and left New Mexico and stopped making payments on the NM home. I was able to obtain a quit claim deed from her so I could work with a realtor to try and short sale the property.

The home went up for sale. A cash offer was made and countered by BOA in May 2013. The counter was accepted by the buyer but the purchase didn't go through for some reason.

The home was foreclosed upon and my understanding from BOA was that I didn't owe anything for either mortgage any longer but that my credit would be negatively affected for 7 years from the date of foreclosure which was May 23 2013.

Yesterday I received mail from an attorney's office in Texas where I now live saying that they represent Green Tree and that I owed 24.5K on the second mortgage which they said I had 30 days to respond to or they would advise Green Tree to "take whatever action is necessary under applicable laws to protect its interest, including but not limited to the filing of a lawsuit."

Their letter stated that their notice was being presented in accordance with the Fair Debt Collections Practices Act.

I called Green Tree and they said that BOA did own both loans but that they still had a right to come after me for the 2nd mortgage as only the primary mortgage was satisfied in the foreclosure.

So here are my questions:

1. Can I be sued for this 2nd mortgage in the State of Texas since the home went into foreclosure?
2. Does Green Tree have the right to sue me for the 2nd loan?
3. I don't have a lot of money and I've seen where folks are offering 5-10% of the loan amount to settle. Is this advisable? I could probably ask a family member to help me for that amount but not much more.
4. I've read online that any settlement should be in writing and that it should reflect a full payment and that no one else will come after me for this loan. Is there a specific wording that should be in the settlement letter?
5. What should my next steps be? My communication with my ex-wife is mostly non-existent and I doubt she can pay anything, or would be willing to.

Any help is appreciated.