My husband owes around 5700 back child support. The child is over 18. He is finished paying the monthly court ordered, and is now just paying the back that comes out of his check every 2 weeks. We got a letter saying there were going to put a lien, can this be done? Even if he is paying.

Yes, it can be done and typically is done. Back child support is just that ; back support, past due. So if you decide to sell your house before it is all paid up, the balance of what's owed will be paid out of the proceeds at settlement.