If you have backups in your shower the blockage's downstream from it. You may snake from one of three locations.
1) Remove the strainer and snake through the shower trap, After working around the return bend of the trap put out 15 more feet of snake.
Pull the toilet and snake from there, **OR**
3) snake from the lavatory roof vent, Put out enough snake to reach the base and 20 feet more. Good luck, Tom