From what you said it sounds like you have two different wheel widths, {15x7 6-5.5 }, which doesn't add up to running the same size on all four.
I would run as wide as I could on the front and the same on the rear, if you want all four the same.
I don't know how you use your truck, but things to consider when you go wider up front are how it will affect steering and handling.
The wider you go up front the more you have to deal with what the road gives you. Without PS it's harder to steer with wide tires up front, especially when parking.
Dan
