Turn the wheels all the way to each side of the truck and check the clearance at the front and rear of your current tires. Then check the added radius and width of the new tires and you will know if there is going to be a problem.
-18 and you for sure will rub. I'm running a -12 and I had some trimming to do leveled at 2.5in. Had to cut 3in off the rear crash bars so I could flip them and then some light trimming on the plastic block that's under with liner