I have a Moen shower with toe tester, when filling the tub, if I turn on the water to more than half of the full water pressure, the water starts to come out of the shower head as well as the spout without pulling the diverter. The diverter is still in the tub position. How can I fix this, and does this mean that there is water running behind the wall?

First off I have to know the house PSI. Purchase a hose bib pressure gage,( see image) from your hardware store and attach it to your outside hose bib.
The average house PSI is 45 PSI. ^9 tops and 80PSI may be too much. What's yours? Back to you, Tom