Shared Custody

In 2002 my wife asked for a divorce in Oct of that year we agreed on shared custody the kids spend a week with me and the next week with their mother. Now the kids are 18 and 16 with the 18 year old being a senior in high school. Their mother moved to another school district and the kids are with me 90% of the time. The kids go to their mothers now and then for a night or maybe two but for the most part they stay with me because of school and friends here. I enjoy having them but at this point in their life providing cars, insurance and meals at times is tough. I am still paying their mother support weekly I have asked her to end it and I have no interest in making her pay or changing the custody order I would like to have the money for their needs here. Their mother does give them money for lunches at school and for other things at times but day to day things she is not available all the time unless my 18 year old drives to her house to get things. So my question is do I have a right to end the support order?