In general you want to run the same front and rear, yes. But the difference between those sizes is less than a half inch which is within variance of unevenly worn tires of the same size. I think you could get away with it.

But unless money is really really tight, just buy some cheap NEW tires of all the same size or get your 31s mounted on the wheels.

90 pickup, 22RE, stiff-ass Ranchos, 2.25 catback, 4:1 header, conical intake, 15x7 ICW alloys, 32x11.50 Dunlop Mud Rovers, 4.56s w/v6 rear

-Farm truck until school is done...then plans

2000 Honda CRV EX 5 speed

-Driver