Ouch! I'm hoping against hope that you don't need new field lines, but based on what you've said so far, that's what you're looking at. I had that problem shortly after buying my 1st house...fortunately my fixtures drained OK, but we were the 1st residents to use a washing machine in several yrs, & every time it ran, so did the back yard. Had the honey dipper truck out to clear the tank, but yrs of neglect had plugged the drain lines (funny how nobody tells you that septic tanks aren't maintenance-free until it's too late!), & estimates were $3000 & up (in early 1980's $$$) to dig up new lines. I was able to stall for 2-3 yrs, as they were just getting the sewer lines out to my neighborhood, lucky for me no one from codes ever looked at my back yard.

__________________
Plug me into somethin'

If the theory does not conform to the facts, then the facts must be discarded.

No good deed ever goes unpunished

Never argue with an idiot. He'll drag you down to his level, & beat you with experience.