I was in prison for 8 1/2 years and I was wondering if I have to pay back child support if they were on welfare. Also it's the child's grandmother who has custody over the child and she is only asking me for child support and not the child's mother. What should I do? Please help

You are responsible for all child support... they were probably getting welfare because you were in prison and thus weren't getting any financial help from you... being on welfare has no bearings on child support... except welfare will seek you out to pay child support so they don't have to keep supporting the child.

Did the mother reliquish her rights to the grandmother? Or is the mother just living with her?
For this, I would consult an attorney.

No the child lives with her grandmother away from her mother. And the mother gave custody to the grandmother while she was in prison

Well if you both (mother and father) still have your parental rights, and the grandmother has custody, I would agree and say you both should be paying the grandmother child support.

But again, I would consult an attorney.

Ok, you are liable for child support according to the court order. If the mother is supporse to have the child, and the child lives with someone else, you may have a case against the mother in court. But you have to pay what ever the court says you have to pay, and you have to pay the person the court says you have to pay, until you go to court and get it changed.