There is noticeable backlash between propshaft and wheels Les which I believe is down to crown wheel and pinion clearances so normal. When reversing slowly I can often hear this backlash especially with lock on but put this down shaky clutch foot.
Have you tried greasing propshaft UJ's? The rear one by diff is easy but forward ones require some ingenuity to get a grease gun in. I believe John V6 had to make an access hole in rear of transmission tunnel to gain access as no possibility working underneath on his Roadster.
You say you have changed diff oil several times but with what? You must use oil with LSD friction modifier included or add it to existing.
