At my last house we had a septic tank with a leach field that worked fine for about 8 yrs, house was built around early 60's, we started having problems with water backing up just like the OP, long story short, I paid to have a new leach field put in where the old one was, the original was about a foot deep so there wasn't enough of an area to allow proper leaching in, made the new one about five feet deep and never had a problem for the next ten years when we sold it.