you need to join the box back together, remove the 5th gear selector so that you can put it in 2 gears. Then undo the big nuts on the end of 5th. If you want to pull the diff out now is a good time to loosen the big nut under the offside shaft seal.
5th gear is stuck onto the end of the 172 box, so you will need a puller to remove it - take care, the gears are brittle.
Once the gears are off the end it should all come apart.
They are a real bugger to get back together and get all the detents etc right.
It's a few years since I had one apart, so take all that with a pinch of salt