How do i stop/repair this erosion problem?

With gullies that deep it sounds as if the land is poorly compacted fill and/or you have a lot of water. You could divert some water away by constructing a French drain, but that's a lot of work. Erosion control blankets aren't cheap, but another consideration. Native gres and wildflowers control erosion, but are not always easy to establish. A good local landscaper could tell you which plants work well in your area. Sod won't thrive if you have a base of hardpan clay or stone.