A 2wd will need a stronger diff than a 4x4 as only 66% of the torque is going to the rear on a 4x4. That is also the reason why a 2wd has a 7.5" and a 4x4 has 7" as standard. The tyres will spin at the same torque, as that is just the grip limit of the tyres, for which it doesn't make a difference whether it's 2wd or 4x4.