Whenever it rains hard or for a prolonged period I get water back up into the septic tank. I also cannot flush toilets in the winter when the growing period has ended

Your leach bed's overloaded with water to the point of saturation. Time to call in outside help and figure out a good spot for either a new location or extending out the existing leach bed. If you don't have the room click on back and we'll look into other options. I got to warn you! None of this is going to be cheap. Good luck, Tom

speedball1: re: leach bed back up. There seems to be little room to extend in the current location in the back of the house, but may be room on side.

Tom is right on all counts. Once a field is saturated extending a line is not the fix needed. It is very likely that codes have chaged and you may be required to come up to current codes very much different than when you system was installed.