From your informaion, this would seem to be a classic problem where Septic Scrub can help you. Your field must be anaerobic since it was "under water". Since you have had the tank pumped and water flowed from the field back into the tank, you have also had the field pumped. I do not know if all of the backflow was pumped, but if it was not, it must be pumped until no further backflow occurs. Septic Scrub treats the biomat around the laterals. It must be able to flow outside of the laterals to the field. Obviously if the field is back flowing into the laterals, this cannot happen. 

You should also ensure that the tank did not overflow into the drainfield. If the laterals contain sludge, they should be jetted before Septic Scrub treatment. This is a high pressure wash of the inside of the lateral pipes. There is no treatment which I know of which will dissolve this sludge.

After this is done, you can add Septic Scrub to your distribution box. While I cannot guarantee that Septic Scrub will restore your particular system, we do restore most of the ones we treat. I do know that the product will react with the anaerobic buildup in the biomat. In most instances, this will also open the field to improved flow. This treatment requires four bottles of Septic Scrub. This cost effective treatment is much less than cost of replacement which you mentioned and the results are immediately evident.

If you wish to discuss your situation further or determine where you can obtain Septic Scrub, please call me at (888) 352-7226.