please can anyone help

I have a strange problem with my MY99 UK Turbo, From cold the car starts and drives faultless and all is ok even when hard accelerating. After approx 1hr of driving I experience a banging and knocking when on full lock, It feels like it is coming from the rear, as if the diff is damaged. There is plenty of oil in the diff as This was the first thing i had replaced when this fault started. Any advice
It just seems strange that it only occurs after a long duration of driving.
I am going to get hold of another rear diff to try exchanging this out.
One further question is do I have a 3.54 or a 4.44 ratio diff, I have checked all the drivetrain charts out and cannot seem to pinpoint which one i require.

 

bttt,

 

I hope that this will help you out.

 

I have a diff available for your model/year in good working order.

£150.

Drop me a PM if you're interested.