You sure that your septic system is not just frozen? Do you have anything that is providing a constant flow of water to the system (slow trickle?) that can be a cause of freezing up on the leech side of things. You may want to do a week of doing laundry at the laundrymat, limiting water usage (if its yellow let it mellow, brown flush it down) etc. Try to only do things that will add hot water into the system and as little as possible. Also add in some extra bacteria to the system to see if you can get some exothermic reaction going in there to help warm things up a bit (hopefully your drain cleaning crystals didn't kill your current bacteria).
If I remember correctly know you had some problems earlier this year did you have your leech field replaced? is it possible you have solids in your leech field?