Urgently need help with E53 engine vibrations when driving
 

Hi everyone. I am writing from Singapore.
I drive a March 2005, X5 E53 3.0i with a M54 engine. Mileage is 110,000km.
Over the past couple of weeks, the car has been giving out engine vibrations when I drive over 80km/h. It moves off smoothly and once it hits 80-90kmh, the vibrations. I release the accelerator and rev up again, and the vibrations are still there. The steering and accelerator also vibrates. It feels like something has come loose very severely.
My car is serviced regularly and used to drive beautifully.
I went to my usual authorised workshop who told me the engine mounting is faulty, and replaced both engine mounts. Still, vibrations are there. Then they change the spark plugs and ignition coil. Vibrations still there and still as bad. They plug the car into the BMW diagnostic machine but can't detect anything.
I tried going to various other mechanics. All can't detect anything. Some tell me my automatic transmission needs an overhaul. But they don't guarantee it as when they test the car, the transmission works fine.
Would appreciate any help of advice from fellow E53 users out there. Love the car, but am saddened by its state now.
Thanks everyone.

are the wheels balanced ?? did you get new tires recently??

yep, changed the tyres and balanced them. Double checked the balancing again. The vibrations come from the engine, only when I am pressing down on the accelerator

Did they check the Guibos? How about the driveshaft support bearing? Any sign of decayed or cracked rubber, these should be replaced...even if that's not the issue.

I'd also want to closely inspect all axle shafts. A torn boot could give a hint at a failure, but the joint can go bad without tearing the boot, (just as, if caught early enough, a torn boot can be replaced if the joint is good).

What are Guibos. They said driveshafts look ok, but there was no mention of bearings. Neither was there mention of axle shaft rubbers wearing out. Will get those checked tomorrow. Thanks.

Guibos are the big rubber doughnuts in the driveshafts. Technically, they are called a "Universal Joint", but anyone who's been around BMWs for a long time will know them as a Guibo (clutch). They certainly bear no resemblence to any conventional universal joint.

The driveshaft support bearing also has a failry thin rubber ring around it that if torn, will cause those vibration problems.