Improperly inflated tires are bad. I think everyone knows that.
But did you know that when the left and right tires have unequal pressure in them, it will cause your truck to drift (pull) to the side with the lower pressure?
It only takes a difference of a pound or two. You can't really get the pressure equal using the typical gas station popup gauge. Use a good digital gauge, one that reads in 1/2 pound increments.