These are notoriously difficult to fix. Your best bet is to dig up the outlet baffle on the septic tank and to find the distribution box, the junction where the different drain lines separate. The ususal problem is that drainfield tile is laid with straight tee or right angle pipe and it is difficult if not impossible for a root cutting blade to turn these corners effectively. Still it's worth a try a few hundred dollars may be better than the thousands a new drainfield may cost you. You also may want to do a percolation test on the soil around your drainfield. Perhaps soil conditions are too wet to take the load from your gray water.