If you search for CV Joint I did a write up on removing the axle shafts. ( no need to undo the big hub nut though) once both sides are out unbolt the front propshaft, drain the oil and unbolt the diff , I recon 2 hours and pretty straight forward.
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what usually makes either diff fail is the centre viscous coupling seized. the front diff will normally let go first as this is the one that takes the most punishment from the steering action.
I would certainly check the couplings is operating correctly before you drive it after fitting the new diff.
From my experience the CV joints will give up before the diff if the viscous coupling becomes solid. This is from owning 2 RR Classics not p38 ownership. They will just shatter internally. The tyres judder on full lock which should be enough warning. You then get the bang if you dont do something about it.