Hire an experienced matrimonial attorney who can bring an enforcement proceeding in the appropriate Supreme Court. Just because she can't sell the house for as much as she wants, so long as it can sell for enough to pay off the outstanding mortgage and the amount she owes you she is required to do so. You should not fail to pay your child support or deduct the amount she owes you from your payments. Doing that would put you in jeopardy of being brought into court for failing to pay your child support and having your driver's license suspended. Your obligation to pay child support is independent of her obligation to pay you.
You really should speak with an attorney to discuss what your best options are - in some instances an enforcement motion may be necessary but, if your wife has other assets, you may be able to have your attorney issue an income execution and obtain the sums due to you that way. Whatever route you take, do not withhold child support - that is a court order and failing to follow it is contempt of court.
This advice is not meant to create an attorney-client relationship and is a general answer to the question posed.