GLA 220D 4Matic Rear Diff Issues 2018 32k miles

[MustangGT]

We recently acquired a GLA 220D 4Matic and have noticed a low-speed issue with the rear diff. It appears to be 'wound up' and releasing. A local mechanic friend said it could be a number of things and I am asking the combined wisdom here what it might be. For me there are 2 contenders:

1) Rear tyres are at about 3mm each, fronts more like 6mm. Is that enough of a difference to cause the drive train an issue?
2) Oil in the diff, could it need changing? Perhaps a more 'slippy' oil would fix it?

Many thanks in advance!

[mwstewart]

It's most likely the oil. Change it - original spec is fine.

[MustangGT]

Thanks, will try that as a first response.