If the growlers are not meant for something pressurized like beer or soda, then yes you're running the risk of having them explode.
All is not lost, though. Let the secondary fermentation finish up without building pressure by replacing the caps with an airlock, or just loosening them enough for the CO2 to escape. In a week or two you'll have two jugs of perfectly good, but flat, beer. Likely you will also likely have accumulated enough empty bottles to transfer it into. Make up enough priming solution for a gallon (probably a bit under an ounce of sugar mixed with 1/4 cup water), boil it, add the flat beer from the growlers, and rebottle.
If you drink at the rate I do, they'll be ready about the time the previous 4 gallons are gone. :-)