think you might have to bite the bullet and pull the van off the road- behind the cover is the end bearings for both shafts-pretty common for the ducato to have box problems seemingly.
yours sounds like more than one problem though-the longer you run the worse the damage will be inside the box-better to pull it when you can drive to a garage of YOUR choice rather than breakdown somewhere inconvenient.

trying to diagnose it from noises alone is pointless mate-could just as easily be a clutch problem too as well

One thing that springs to mind when you pull the driveshaft without first draining the gearbox half the oil in the box can spill out, did they top it back up.

Are you sure they changed the whole shaft and not just the CV, either way could be some spurious junk.

You can see the exploded parts diagram on fiatdalys
https://www.fiatdalys.lt/en

Update : turned out there was an issue with the gearbox that was made worse by the fact that when the driveshaft was replaced they didnt refill the gearbox up with oil. Only 1/4litre came out.
So ended up having to replace the gearbox, which cost me £400 and three days of my freind n myself life struggling to do on a driveway.
Gearbox price was a bargain as cheapest ive seen was £800-£1300.

Change your garage pronto , right load of cowboys not even doing basics , if you can prove they have done crap job i would suggest small claims court next ..!!!!