A 2wd stock bay will get suprisingly far offroad. We used to camp at Castlerigg Farm in Keswick and were always put at the top of the muddy hill to keep us out of the way. The bays would always make it to the top of the hill but all the T4s got stuck and some of the T3s had problems too. 50/50 weight distribution isn't much use offroad with only 2wd. The blue bay in this video is 2wd with just bigger tyres and apparently it made it everywhere the T3 syncro did. The shasta show trip the splitscreen bus guys do every year proves how far your can get a bus too. The locking diff from a syncro can be retro fitted to a 2L 091 box pretty easily, though using a syncro centre section of the box makes it even easier as the boss for the diff lock actuator is already there. A locking diff would make a bay pretty capable off road, though being able to say you have a 4wd bay can't be beaten. If i could find a solid double cab bay shell i'd be seriously tempted to build one with syncro running gear just for Marmites and giggles.