Bongo Fury

On a recent drive back from France I started to notice a noise that I initially thought was from the road surface. It is a whine / whirring sound that is only heard when moving and increases with speed (like a bus getting going). I took my Bongo into an approved site and they changed the f/n/s wheel bearing (not sure why they didn't do the other side) but this did not solve it. They have then stated that the differential oil is 'very black and smells hot'. They replaced this but still stated that the noise is coming from the front differential gears.
I have been quoted £1,650 (+VAT) for a new differential from Mazda.
Can anyone give any idea about whether this is needed?
(my Bongo is 1999 4WD with 53,000 miles. The garage queried the front and rear wheels being different sizes but I note from here that the sizes look correct. I've always had a shake on the steering wheel at about 65 - 70 but nothing else of note)
Thanks

one of the main factors to cause this in my opinion is wheel and tyre size.
according to mazda you should fit different size tyres front and back. but most 4wd go for all four the same size (the rear ones).
if you have tyres as mazda quote there is approx a 1% difference front and rear. this is providing all tyres are equally worn, problems can occur when you put a set of new rear ones on, and leave the front well worn ones in place, or viceversa.
the secret seems to be to put all 4 new tyres on together.
i had a problem a while back similar to yours, and i traced it to a bubble on the tread area of a front tyre, only small, but enough to knock the rolling radius out of tollerance.

edit. check the front diff breather is not blocked also, as this can lead to the diff overheating.

Thank you. Do you think that the change of tyres /wheels or removal of potential would resolve this issue or is the differential damaged and in need of a costly replacement?

you could remove the prop shaft and run as 2wd and see if noise goes

my front diff survived without further work as i spotted it early, but some have had bearing failure in the diff itself, and have had to replace it.yours sounds like the later as you say you have done a bit of miliage recently.
you may be lucky and get away with just removing the front prop, but a change of oil in the diff may help. has it lost any oil it usually gets driven out at the driveshaft seals, check both sides. have you checked the oil level.

I had a whirring noise after I fitted new front tyres. As the rear ones were nearly shot too, I changed them very soon after and the noise went away.

I have a 4wd and run all tyres the same size, this means I can swap front to rear to even tryre wear and reduce stress on the ageing transmission.

I think that the oil is ok as it was replaced when the garage checked it, but i'd recently changed a front tyre prior to going on holiday. It looks from your postings like this may have contributed to the problem. Can anyone give any advice about where to get a new differential if need be. I'm trying to avoid the £2,000 cost if possible