one of the crappy TV motor programs had a report from Iceland where they use that method to inflate their huge tyres. They have to deflate their tyres to get a bigger footprint when they are driving in deep snow and quite often the bead and tyre seperate. They use lighter gas!

look under car when in gear with the engine running. If you can see either prop shaft turning the problem is not in the gearbox.You did check the high low lever didn't you? My partner called me out once saying the gears wouldn't work she'd pulled the wrong lever and put the transfer box in neutral

If you want to make sure the problem is in the fuel system delivery run a pipe from a can straight to lift pump bypassing all delivery pipes. It's possible you have a cracked pick up pipe. Does this problem occur when you have about the same amount of fuel in the tank? As the fuel slops about it...