You may have two issues here, the first an abs issue, but the second may well be a usual failure in your generation of xc90 and mine, that is failure of the spline drive to the transfer box.

Simple test is to lay on your back, feet forwards and reach up just in front of the rear wheel arches. You should be able for find the prop shaft, use rubber gloves which help to get a better (and cleaner) grip. If you can turn the propshaft, then the spline drive has failed.

The car will not show error messages for this, but will be very apparent on gravel car parks or in the snow as the back wheels are not being driven and the front will slip.

Not a disaster to get fixed, you will need a new spline drive, which has been updated. The main cost is the labour to remove the transfer box and hopefully the wear has not extended to the transfer box drive as this can become worn if a stripped spline gear is not sorted quickly. Result is a replacement transfer box.. ask how I know that!

I have a horrible feeling that’s what my issue is, under moderate acceleration there is an intermittent (I want to say grinding, but it’s not as disastrous sounding as that) noise from what to my ears seems to be roughly below the back seats? (Although I have been caught out before with trying to pin-point where sounds are coming from with a failed wheel bearing on a Renault scenic sounding as if it was coming from the back) this noise in the Volvo though, it’s almost like a rattling shield over an exhaust, but worse. More like the exhaust rubbing against something. Could well be that. When I first got it the sound was worse. Then I had the top and bottom engine mounts done and it almost silenced it. But it’s coming back.

Mine did not have these symptoms, but it could be the spline gear has just started to slip and transferring noise down the propshaft.

But sorting sooner than later is best, especially with winter coming along.

Cheers

I have had the vehicle looked at, the propshaft won’t turn so that rules the splined drive out, however there are several fault codes:

0094 communication between control modules, communication problems with active on demand couplings.

0109 communication between control modules, communication problems with steering wheel angle sensor, internal fault.

1464 communication with the differential electronics module (dem)

0016 control module communication fault

These were read with a Snap-On Modis Ultra

Need to check the SAS and BCM data streams to see which is at fault when the DTC is raised. Fairly easy with Vida. Also can often identify the actual fault:
SAS angle changing randomly as wheel is turned - optical sensor(s) blocked.
SAS 180 degree out - SAS needs clocking - probably because someone tried to fix and put it back wrongly.
SAS slightly out - calibration issue.
BCM angle/speed regular or cyclic variation - ring on that wheel.
Complete lack of data on PID - bad sensor etc.

I had a faulty DEM and I'm sure it triggered the ABS warning light. There is no DEM warning light on the dashboard so this would make sense to initially warn drivers of a problem. Maybe someone else can confirm this.
I also see you do not have any ABS warning codes from the snapon scantool - this may also support my DEM theory.
My failed DEM through up all sort of communication issues with other modules that the DEM speaks to - I'm sure the DEM will speak to the steering angle sensors inorder to control DSTC (Dynamic Stability & Traction Control).
At the end of the day best to buy Vida Dice as it will save you loads in the long run- cheshired5 - a forum member does a good deal on this and is extremely helpful. Or you can pay Volvo/Indie to scan it with Vida Dice for £50/100.
In the meantime I think there is a way to use the oddometer button to get more detailed fault codes- does any member have a 'how to' on this?
If it is the DEM that is your problem or one of your problems, Lucaz(another forum member) has a company that repairs DEMs for £140 with a lifetime warranty. see https://volvodiagnostic.com/