Vauxhall Brava (isuzu inside) Prob

I have just been for a drive in a mates ex AA Vauxhall Brava 4*4, and have noticed a problem. When 4wd is engaged, the rear wheels seem to drag,
almost as if the front wheels are pulling the vehicle faster than the rear ones. The vehicle is fitted with free wheeling hubs to the front wheels, but there is no switches in the cab to activate them(if that is how u do it!) Any ideas as to what the problem is? and is it expensive?
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