It really depends on how far you would need to drive. The diameter of the 235/75 is about 4" less than the 33's you are running. I'd use the 31/10.5/15 as it's only 2" less and then use it only as a limited emergency spare. You can take the truck around town, but limit as much as possible highway speeds and distances. Most of the speed difference will get absorbed by the differential, assuming you don't have a limited slip setup. If you do have a limited slip setup on the affected diff, that's where you'll potentially ruin things quickly.
AWD cars typically run into issues because those have fancy electronics and ABS systems that get confused if a tire is going a different speed. Plus, there is often a clutch or other torque vectoring system that can get hot and destroyed in newer AWD systems. The 203's are pretty tough, the chain/gears may get worked a little so just take it easy and get the right size tire on asap!
Finally, it's really not hard to convert the NP203 to part time and put lockouts on the front hubs. You'll get better fuel mileage, reduce wear on front drivetrain, less harmonic vibrations in the cab, and not have to worry about mismatched tires up front if you had to mount a spare! Of course you loose the AWD, and some people really like/need/want that. Just my $.02.
Have a great one,
Alan