My 2011 Nissan X-Trail recently had the front wheel misalignment corrected and two new Avon ZX7 front tyres fitted at my local branch of a national tyre franchise. The rear tyres were part worn but the tyre fitter didn't warn me of the perils of mixing part worn and new tyres on such a four wheel drive vehicle. I have to admit to being naive on these matters. Within a few miles of motorway driving a speed dependant hum developed which I wrongly attributed to the new tyres. Unfortunately the noise got worse and my journey came to a premature end when I had to call out my breakdown service who diagnosed failure of the rear differential. The vehicle was recovered to my local Nissan garage who have informed me that a new diff will cost around £2500 plus fitting. They have measured the front tyre tread depths at 7.59mm and 6.36mm and the rears at 5.48mm and 3.39mm. I'd appreciate your opinion on the possibility of the front/rear mismatch causing the failure of the diff and whether the tyre fitter was in any way negligent in failing to warn me of the possible consequences. If nothing else, it would have been in his interest to sell me two more tyres.