To all contributors thanks very much for your great advice !
From that advice and doing a bit more research myself i am going to start with the near side rear half shaft ( i`ve read that this side is the most common to fail ).
Regards using second hand original parts is another very interesting point to consider " thanks 4X4 BY V8 " it appears the usual new part suppliers parts have greater failure rate.
Don`t know whether there are different types of half shafts ? Old Betsy is a 1981,88", series 111 with overdrive,could anybody enlighten me.
Kind Regards.