Pulling out the half shafts is dead easy.
Remove wheels
Remove brake drum cover
Remove brake shoes - - I took a photo so I could put them back right. All very easy.
There are 4 bolts behind - undo them
Now pull the whole assembly (obvious when you look at it) and the half shafts, along with the square plate at the back of the brake drum (that you removed 4 bolts from) will pull out. You might need to bash with a rubber mallet.
You only need to pull them out around 6 inches but you can't do any harm pulling more. The shafts will come out with their wheel bearings in place.
You will also have to disconnect the horizontal suspension arms and one shock absorber bolt from one side to allow the whole axle to push back a bit - this gives you room to get the diff out as it is going to have to come forward before it will drop out as the gears live inside the axle housing. From memory I disconnected both horizontal bars on both sides and then one shock absorber on one side, this allowed plenty of movement of the whole axle.
Now the diff will come out. (clearly you have to undo the drive shaft flange bolts 1st)
I undid the diff casing bolts leaving the top one in place till last, then let it drop into a large baking tray (there will be a lot of oil - I recommend loosening all bolts until oil starts leaking, then leave it a while to drain)
Your ratio is simple to work out once the diff is out. Count the teeth on the small pinion gear at the bottom, and the teeth on the large crown wheel - and that is your ratio. - You them make it more usable by dividing the big number by the small one. eg 9 teeth pinion and 32 teeth crown = 32:9 or (32 divided by 9 = 3.55) 3.55:1
pm me and I'll give you my phone number and you can call me if you want to run through anything. I was new to all this and it's actually rather simple. The trick, for me, was to make sure I have a way of handling the oil spillage, and having good axle stands so I was happy being under the car (I also put the rear wheels under as a backup). The rest is just a matter of taking your time and taking it one bolt at a time :-)
p.s. I have a 16v 1600 engine that revs to 8.5k and I had a 3.77 diff which was wrong for my car. I now have a 3.89 which is much better
The ebayer I bought my diff off promised to refund me if it was a bad diff but he seemed confident it was ok and clearly knew what he was talking about. There is of course a risk, but I decided it was worth a punt as a recon was looking to be around £600. I figured that I could "try" 5 diffs from ebay for that :-)