Help - car won't reverse!

Hi, I have a problem with my A8 1999 4.2 Quattro Sport. It worked just fine till yesterday when the problems started:

I set the car to R and when pressed the accelerator the car was shaking like I would have been pressing the brakes on and off very quickly.

Ok, and now the car has stopped totally going backwards. When I shift to reverse, it goes backwards a little bit, but if I touch the accelator - it's like having the car on NEUTRAL! Nothing! No response.

What's wrong? Is this fixable or do I need a new gearbox? Any ideas?

Thanks for your help.