Yes, put the studded tires on all fours. But don't blame the tires for the total mpg loss. You're pushing through snow and that takes more gas no matter what tire you run. It's colder, so you probably let the truck idle longer to warm up... needlessly. You're probably driving slower and not at optimal mpg speed and also taking longer to get wherever it is you're going... not to mention the much slower city driving and idling.
And remember... Winter driving is fun!
