Sounds like a drain field that is in need of repair. Depending on the age of the house it might be time to replace it.
One way to check this out and double check the word of the septic service company. Is when they pump your tank (which they will want to do in order to diagnose the drain field) make them do pump it from the outlet end of the tank. When the pump the water level down, listen for water running back into the tank if this is the case then it is time for a new drain field.
You can get someone to do a video inspection of your lines to see if they are clogged or damaged but more then likely it is already to late.
The only other way to test this without servicing your tank is to hold off on laundry and as much water as you can for a 2 or 3 days and check the out let end of the tank for the same problem. If it is still over full you know the problem
JC Septic Service