That precise thing happened to me two weeks ago, Sigurd.

They said the rear differential on my car needed to be replaced for 14K USD. When they did that, they then said the problem still persisted and the front differential would have to be done for about 18K. Needless to say, I was a bit enraged.

In the US you can call AAA (if the shop is a member, which I think most are over here) and have them arbitrate the dispute. They ended up giving me the car back with the rear dif done for free. I was expecting the problem to still be there like the mechanic said, but lo and behold the car was completely fine. I guess they figured I'd be stupid enough to just let them go ahead and replace the front differential without me even coming down to check the car out first.

This worked out pretty well since I basically got the car fixed for free. (I'm still waiting for the other shoe to drop. The wheels will probably fall off later today since I'm here talking about my good luck.)

Anyway, good luck. Just another reason to move to a city with good public transit for me.