Help with a acceleration/vibration problem

[Locke S80]

We have a 2000 S80 and really love the car. We have a problem however, when you accelerate there is a vibration in the pedal and the wheel. Feels like it is coming possibly from the transaxle, not sure though. We have taken it to the dealer (not a volvo dealer) where we purchased the vehicle as per the extened warrenty and they are not able to find the problem, said it could possibly need an upgrade of the programing of the chip in the engine, said it would have to go to the volvo dealer . Now before they charge me a gold mine in cash for repairs I want to find out if anyone has had this problem so I have ammunition when it comes to the talking to the volvo dealer and then the bill. HELP!