Dean,
If you have clearance issues on the rear, then you obviously can't get the same diameter tire on front and rear. Therefore you can't use 19. 5" wheels because the rolling diameter must be the same for front and rear.
Actually, you can use them. You just can't use them in 4WD. If you disable your transfer case shifter (remove the connecting rod, for example), you can leave the truck in 2WD and use any tires you want. They don't even have to match right-to-left, although that will cause some problems with the differential turning more than necessary.
If you engage 4WD with different tire sizes front and rear, and if you're lucky, the truck will just stop as the drivetrain binds up, or perhaps one or two tires will skid periodically as you drive. If you're not lucky, something will break.
Don't do it.
Loren