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Yep, it's easy. Just make sure you use the proper size sockets so that you don't round off the bolts. And you may need to move / roll the Disco a few feet up or back after getting a couple of bolts to rotate the driveshaft and give yourself access to the other bolts/nuts.
PS - If I could give any advice. 100% avoid using an off brand or Britpart one... A Britpart one I put on lasted 3 months with one very light offroad trip. Swapped with genuine LR and fine. It stretched, cracked, and eventually failed going 80 on the freeway, and two of the bolts flew off clanking down the road. I was not psyched. The genuine LR are about $130-150, the cheap ones are $40-50.
The bracket with the weight is a vibration damper (found on many solid axle LR's, the older ones mostly) for the axle, which apparently helps avoid a resonance or some other vibration that can occur. D2 has it on the rear, but not on the front. Interestingly, my 88 RRC has the same thing on the front, but not the rear! Leave it on.
What is the little extra part that comes with the kit? looks a bit like a bearing insert or something.
There is a centering pin that can wear out. It is a friction fit part that inserts into the flange piece for the diff.
About your donut, surface cracks do not indicate failure. Looking back, I probably would not have changed mine if I knew what I know now.
You also have another option, to convert to a four hole flange with a u-joint (harvey spicer joint). The u-joint is not as smooth on-road, but more reliable off-road.