toilet water coming through the shower drain
Before opening clean-out plugs and snaking the drain, you might try a good non-caustic enzyme-based drain cleaner like DrainCare first. You just run warm water, pour in the DrainCare, let it eat out the organic gunk overnight, and then flush it down with hot water. I usually start that with a kettle of boiling water. Sometimes this works.
If it doesn't, then you have to snake the drain.