Welcome to the Forum, SiriusHog:
Your backing up indicates a partially cloged or blocked sewer line. If your sewer is anything besides PVC plastic there is a good chance some tree roots or bermuda grass roots have found a tiny leak and have grown into it. The showers and toilets are the lowest fixtures in the house, so they will show the back-up first.
You need to dig up the sewer "test tee" just outside the house and use a mechanical drain cleaner on it (roto-rooter). My house has vitreous clay tile sealed with cement for the sewer. Well, the cement always cracks and allows some leakage. Any where there is a leak, the roots will come in and they continue to grow until you have a full blockage. After I had cleaned mine 3 times I wized up and started flushing a cup of salt pelletts (water softener pellets) every week or two and haven't had a problem for 3 years now.
This time of year and yours being only partial blockage, you might try the salt deal first, say a cup every week for a while. The idea is you have to kill the roots then they have to rot off. A bag of pelletts at less than $5 shure beats a $75 service call form a plumber. Let us know how it turns out.