1997 Mitsubishi Pajero whirring and juddering about 50mph - transmission ??

I have a 1997 Pajero 3.5 V6 automatic. Recently a whirring noise started, it seemed to be front neraside wheel bearing type of noise. Took it into local garage and they diagnised it as this, as the noise seemed to go when you turned sharp left. So got it changed, but still have got the whirring noise. Just in case something silly like tyres, these were crosssed (all same make). Still the same noise. Now the noise has got worse. When you start the engine and are not moving the noise is not there. As soon as you drive off the noise (like a whirring noise) starts. When you go above 50 mph, the noise gets louder and vibrates right through the steering quite badly.
Took it back to the garage and they now reckon it is the bearing inside the transmission unit at the front of the vehicle, and have priced it up as a 500 pound + job. My concern is that if it isnt this, and perhaps something in the steering am I wasting my money?
They also reckon that they have never had to change one of these before and have seen lots of Pajero's, so again bit wary.

HELP !!