For a GU with 2" lift you could fit 35" on a negative offset rim easily by only just removing your mud flaps before they get removed. As for gearing it depends on the intended purpose of the vehicle.
Snicko for you to fit 33"s all you would have to do is buy them and get them fitted. BUT check the distance between the radius arm and the tyre at full lock with your 31"s on and just add roughly an inch to see if they will rub. If you think they will rub then you might want to consider a negative offset rim to accommodate the larger tyres. My GQ with 33" tyres and the standard 0 offset rims and 2" lift scrubbed at full lock left on the radius arm on the LHS. Yours may do so also. Did you fit castor correction bushes/drop arms to fix up your diff alignment when you lifted also??? As this could eliminate the problem of rubbing on one side.
There are many things to think about. But I would just test the full lock thing and if you reckon they will fit then go for it.