water backing up?

we have the laundry room right next to the lower level bathroom, no matter what water (except the washer) we run, it backs up into the shower through the drain, bring with it some nasty tagalongs and a very unpleasant smell. no matter where the water comes from, it just kicks back up into the tub, once more, except the washer, we've snaked the main pipe going out to the septic tank, and poured every chemical you can think of to unblock it, the more we use, the more it seems that it isnt a block. about a year ago, we had to get a new septic tank, the old one was over 30 years old and fell apart, and thats about when this started acting up, were looking for as many answers that doesn't mean tearing up our lawn more than it has already been, the grass has just grown back. so please, if you have any suggestions, id appreciate them, thank you.