Is there any better way of doing this than just guessing and replacing random parts? My Audi has a vibration when the drivetrain is under load at around 60-80mph. It has been to Audi twice now and they have spent 2 weeks and the best part of £1k of my money looking at it and running it up on the ramp to 70mph to see where is vibrating.

This time say that they could see the rear diff vibrating and have done a couple of things to the diff/prop/rear driveshaft which they are confident has 90% cured the problem. To 100% cure it they list a new diff at around £2k and new driveshafts at £200 each (all plus VAT). At first it did feel better but now i'm not sure (the problem is only really obvious on the motorway when you are constantly around these speeds)

I am confident that the problem feels like it is coming from the front end and as the steering wheel stays still it is a driveshaft or front diff problem. But I don't want to throw money at new parts that aren't needed! Is there any better way of telling what is wrong?